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There is an army that destroys and massacres most kingdoms (not all). It’s not for power, money, resources (including land) or slavery. Why would they do this?

Religion?

Different ideologies is always a great reason to get invaded and massacred and has been used throughout history. If your religion demands blood sacrifices, many war prisoners would be needed on a regular basis.

Sport.

Many of the animals in Africa are actively hunted for the sport of it. We kill them for nothing more than the opportunity to have our photo taken holding a rifle and standing beside something bigger and deader than us. Imagine an army that has so little regard for foreigners that they see them almost as another species, and see military conquest as a safari.

Such a culture in time would even end up with a hierarchical promotion model within their military based on gamification. To get to Corporal, you must make X kills, to get to Sergeant, Y kills, etc. In such an environment, war and massacre is actually required for advancement and as such, an army containing a sufficient percentage of ambitious soldiers will be constantly looking for new targets to satisfy the need for enemy bodies to fuel advancement.

The problem with such a model is stopping it. It would be voracious in its need for conquest and massacre, wouldn't consider the strategic benefit of lands or people, or even care too much about the state they represent. The enemy would be found everywhere simply because that's how the next rank is achieved.

If the leader/king/general of the army is evil, or somehow not in the right mindset (like they have psychopathic tendencies, etc.) then they could just be doing it because they're bored or something along those lines, like the way people hunt for sport and not for actual survival or need. Or it could be a show of power (although that could be similar to them doing it for power), like the leader and/or army is bragging of their unbeatableness/power.

Purity.

People from other kingdoms are not like us and don't deserve to be treated as such. They sometimes come over to our land and infect our people with their evil ideas and poor choice of religion and different skin/hair/eye color (or other features) and make little half-breed babies.

We need to stop the infection at its source. Any kingdom that is not worthy of us will be destroyed. Sign up to do your duty, make your kills, and be assured of a place in the world to come.