Zastoupil's answer is 100% the way to go. Using this as an opportunity to create a plausible civil society will pay dividends: your players may tone down their violence when they discover it has costs, and they may take your world more seriously if it feels a little more real.

That's the correct answer. But here's a fun one: redcaps!

From Volo's Guide:

In the Feywild, or where that plane touches the world at a fey crossing, if a sentient creature acts on an intense desire for bloodshed, one or more redcaps might appear where the blood of a slain person soaks the ground. At first, new redcaps look like tiny bloodstained mushrooms just pushing their caps out of the soil. When moonlight shines on one of these caps, a creature that looks like a wizened and undersized gnome with a hunched back and a sinewy frame springs from the earth. The creature has a pointed leather cap, pants of similar material, heavy iron boots, and a heavy bladed weapon. From the moment it awakens, a redcap desires only murder and carnage, and it sets out to satisfy these cravings.

If your PCs are on a killing spree, perhaps they cause an epidemic of these lil guys. It's not as rewarding, probably not even as effective as using your player's behavior as a rich worldbuilding opportunity (as Zastoupil recommends), but it is a whole lot easier. Have fun!

Ask them what sort of game they want, and tell them what sort of game you want.

Some people just want to play a hack and slash game. It's very relaxing to murder things, and that's a perfectly reasonable type of game. You don't, and spent time crafting personalities for your NPCs. That takes time and effort.

As such, you should talk to them. Ask them what sort of game they want, and tell them what you want. If they just murder all your work then a lot of your time is wasted, so you should know whether they are willing to play nice.

Talking about morality is also important. It's a common house rule in many games that you don't wantonly murder civilians for petty cash. It's worth talking about that.

You could impose IC punishments as others have suggested, but going to the source is a more reliable solution.

Disclaimer: This might be less in line with other answers, as I come from a way more hack'n'slayish game style and role play is only done sparse - much like right from the KoDT pages :)

First and most usual way: the solar cannon - or in your case guards, well trained shopkeepers and accordingly equipped shops. There's a reason that pawn shops and gas stations in some areas and other focal points do feature heavy security.

Then there's a more urgent question you may want to ask yourself:

• Is this really a playable setting?


• Is it a world my players want to play/live in?


• Are they interacting with the proper part of your world?

Worldbuilding is often handled as making up a fairy tale story with a way more trusty and positive assumed structure than in real life (happily ever after already built in). While such a fantasy may be enjoyable when describing it, it's easy to forget all the mechanics that are needed to keep a world like that running.

That's why I like when players come in.

Essentially they are code testers, looking for bugs - or in this case plot holes. A shop without any security will draw thieves - at least as soon as real (and independent thinking) humans are involved. Part of role playing, from a players perspective, is to try other behaviours out than they do in every day life. It is their urge to try your setting. They are not actors following a script.

The same way you got an image about the world you create, the players have an image about the world they want to live in. A real game world will always be a combination of both: it is shaped by the background you set plus the actions the player take. Many of their actions were not imagined by you in the first place. To me that's part of the fun DMing - after all, if one just wants people to enjoy his view, he should rather write a book (or game material), but not run a game.

One lesson here could be selecting only players that adhere to your world view and play along. The other is adapting the world to your players needs (and actions). It needs reaction appropriate to the setting. If they rob a squash stand at a farm house, the farmer will be helpless. If they try the same even at a small village, the villagers will react less than friendly - if they got proper information. In the End, even just a few villagers are a good force.

Unless the PCs go all out war, they will end up send away at least, but usually well beaten up, maybe even including tar and feathers. In a large village, even going berserk won't help much. It's often forgotten, that simple weapon stats are mostly meaningless when confronted with a mob or a well coordinated guard force in a confined space.

Then there's the question about if the part of your world, the PC are in, is the appropriate one. Letting a bunch of high-maintenance low-manners mercenaries into a well off low-security upper class neighborhood may be good for some laughs, but almost never works out as intended. They miss knowledge of the inherent rules (and the smell) of that environment. In some ways it's the same question as about having hired the 'right' players. Adapt the setting or change the players.

In some way your comment about sending them to the woods already shows how to move them into a more appropriate location :))

As always, it comes down to communication which depends a lot on your playing style.

One doing a strict DM vs. Player and everything has to be done in character will of course have a harder time to communicate flawed in-character handling by the player. This is much like in real life, when action and words are not the way they are intended - it's hard for the one acting to spot the flaw. Much like someone believing to be a generous and lovable paladin, while voiceing him more like an arrogant jerk. While this may as well be like in RL, it's more often than not unintended and based on the players poor play of a character's traits. Long story short, a roleplay-targeted style makes it next to impossible to communicate on a meta level about how characters are played. This leaves two alternatives:

1. Go along and accept a killing spree by not opening a meta level


2. Do it like a director handling his actors between takes. Pause the game and tell exactly what's wrong and needs to be changed.

In the end, it comes down to what has been said about who rules the play. The players due to their actions, or the world builder by definition. With many interrupts and corrections the world builder's fantasy will come thru and players are just actors - sans the money and fame motivating them to comply.

In a more relaxed play style, where metalevel between players and play of characters are intervened, a DM may as well point out flawed handling and give players a direct choice to think again. In my experience, that will lead in the long run to a better handling and understanding of the players about their characters, even including changes in their characters traits without being forced by the DM. Even more, since the meta part is handled with all (or at least the involved) present, players will start to look closer on all characters, help each other and improve the way the characters are played.

This doesn't stop characters from going on a killing spree - just now they will be way more serious about the implications - and accepting reasonable in context reactions by the DM. If they are playing low life, their city environment will be much like above mentioned pawn shops, pre payed pubs and fenced off shop keepers. Not to mention the many nice ways alignmental magic and change of alignment offers.

Bottom line, PCs are not ruining a setting, but improve its design and features.

Tell them they have options

I keep a Discord server for mine that allows me to communicate house rules, corrections, out-of-game RP sessions for development or non-combat stuff*, and to communicate with them in a dedicated place for things like this. I won't force them to do things in certain ways, but pointing out that they may have other options open to them helps to lead to them actually using those options. As they hear of interesting paths I may have set up for those options, they are more interested to look for alternative ways of doing things. Last session of 5 hours only saw two real fights I think and one of them they were clearly losing!

Online RP sessions

The asterisk above mentions this. Using a discord server, slack channel, or similar, can allow you to have your players wander about town and interact with NPCs, the town, downtime, etc, in a way that is not condusive to combat.

A couple campaigns ago I used this to great effect and ended up creating NPCs based on their interactions with the world that entirely changed the course of a PC or two's paths.

Unbeatable Encounters

Some number of your encounters are supposed to be essentially impossible. If you sprinkle these in every once in awhile you will end up with a chase scene, a new quest, interactions with NPCs to defeat The Thing, or similar.

By putting your players on the back foot you are engaging their other game-senses to figure out how to get around this new problem. If your players aren't the most powerful thing in the world, it's time to introduce a villain that shows them they have a long ways to go. By the villain occasionally making a mockery of them or introducing obstacles; you also make that villain visible, important, and give the players a goal of "Someday we're going to be powerful enough to take that on!"

Puzzles

You can't kill a puzzle.. most of the time. This is a great way to encourage exploration, thinking outside of the box, and similar. Puzzles in this case can include conundrums like "how do we scale this cliff with those archers shooting at us?" "How do we get passed this pit?" "What do the Runes on this door mean, and how can we use that information to open it?"

If you do these well they will feel as fun as a battle without you basically doing any work. My most memorable was a low-level pit trap someone fell in, which they needed to be lifted out of in their heavy armor, and then people rigged a rope system to get them across and whatnot; entertaining and probably an absurd hour or so of effort for what I thought was a throw-away trap.

This is your job

Your players interact with the world based on your lead, in some sense. If you want them to do more than kill everything you need to put them in situations where killing things is not an option.

Terrain, traps, puzzles, unbeatable enemies, characters who interact with yours while clearly not being a threat. Consider the thief who gains their trust and requires checks to figure out he's slowly pilfering their stuff.

I'm guessing none of your PCs picked chaotic assassins, and you don't want to be negative over the fact that they keep whacking every NPC that crosses their path...

1. 
   Literally beat them to the punch.
   
   Example: black blah you're about to sneak behind the shopkeeper when a disgruntled elf slithers from the alley plunging a dirk into his back. He falls into your arms, his comrade steals the item your PCS wanted and to make matters worse a woman fom the inn has just chucked two drunks out and has only seen said shopkeeper flop dead from your arms...
   
   Then they can get run out of town or tossed in jail.


2. 
   Let them attack the NPC.
   
   They could even get that knife they wanted, only to find out the coveted item is actually cursed. Now they have to spend every other day being the opposite sex or that they find out the NPC is a trickster God, who curses them, until they perform some act of contrition. The curse can be any gold they touch turns to ash. Every time they're going to bash an NPC they don't need to bash about, as the DM, roll any dice you want behind the screen and announce they made their Perception or Intelligence roll - then announce an alternative action.
   
   Example: it strikes you, before you're about to strike the shopkeeper, that he might be a more valuable ally in the long run because of his various market contacts. So PC's Name, you want to offer to buy him a drink, and win that knife in a card game and have a good contact, or chance him recognising you're the thug that robbed him?
   

I could suggest opposite than others are suggesting. Instead of punishing the players, enable them. Try building a world where all people have same mentality as the players. In such world, people would have hard time trusting each other. In such a world, finding peaceful place to rest would be difficult. And staying in one place will quickly invite different and stronger group to rob or kill the players. Same with trade. Merchants will avoid players, unless they know them very well.

There might be cities where people don't immediately kill each other. But those cities will have strong defenses and won't allow just anyone to enter. You first need to gain trust of the townsfolk and even then, you can only enter if you dissarm yourself and put on "magic limiters". But outside those cities, there won't be anyone to help the players or provide shelter.

This will have effect of players as they won't be properly rested and constantly on edge of hunger.