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I am very confused by this sentence in the novel/movie of The Avengers I:

"You have heart."

My question is :

1) Why not "a heart" but "heart"?

2) If one has heart, which normally means kind/nice emotions/feelings, then how could one with heart turn to be bad guy? (shouldn't it be someone with a bad heart is more prone to turn bad?)

3) I feel like 'heart' here means something negative, but I could not find a match in the dictionary, as in the dictionary it means: the organ in your chest; deep feelings and beliefs; love; the most central and important part; bravery or determination or hope;(from the Collins). I am not sure which one fits the context?

4)Any other explanations?

The context is:

The stranger, Loki，jabbed his magic powerful spear at agent Hawkeye, said the above sentence, and then, he got Hawkeye under his control. Fury is the head of S.H.I.E.L.D., a secret intelligence aim to protect the world.

Here is the sentence:

Also, the stranger held a kind of spear in his right hand. Set into
its head, a gem glowed the same icy blue as the energy that had
spilled from the Tesseract.

“Sir,” Fury called as armed S.H.I.E.L.D. agents closed nearer, “please
put down the spear.”

The man looked at the spear as if he had only just noticed he had it.
Then, slowly, he looked back up at Fury, and a vicious smile spread
across his face.

He jabbed the spear in Fury’s direction, and a blast of energy from it
knocked Fury and Hawkeye back through a bank of monitors and
instruments. The S.H.I.E.L.D. agents opened fire, but the bullets
didn’t seem to hurt the man. He leaped, spear held high, and cut a
path through the agents. In a very short time, the only people left
standing in the lab were him and Hawkeye, who had just scrambled back
to his feet. Before Hawkeye could unholster and aim his gun, this
strange enemy was somehow already across the room. He caught Hawkeye’s
arm and said softly, “You have heart.”

The tip of his spear touched Hawkeye’s chest, not hard enough to hurt
him. The gem glowed, and a strange expression came over Hawkeye’s face
for a moment. He and the stranger looked each other in the eye, and
Fury was amazed to see Hawkeye put his gun away.

Now Nick Fury really knew he was up against something . . . unusual.
The only thing he could do was get the Tesseract and try to keep it
safe while S.H.I.E.L.D. finished the evacuation and called in some
special reinforcements. Tony had to hear about this.

Fury had the Tesseract in a steel carrying case and was taking a step
toward the door when the stranger turned to him and said, “Please
don’t. I still need that.”

“This doesn’t have to get any messier,” Fury said. He glanced quickly
around, trying to figure the quickest way out.

The Avengers I

Could someone help please?

If you check the Cambridge Dictionary, you will see that heart can also mean "courage, determination or hope".

Given that Hawkeye was the only person still standing (apart from the stranger), determination or courage is probably the intended meaning in this context.

This could be easily a synonym of:

You have courage.

It simply could mean (one of the meanings from Cambridge Dictionary):

courage, determination, or hope

The other answers have the word covered, but do not really explain the context: when Loki says “You have heart”, he is admiring Hawkeye’s strength and resilience, and deciding to put it to work instead of just killing him. In this case, the phrase “You have heart” Is a way of explaining to the viewer why Loki uses the scepter to brainwash Hawkeye instead of just stabbing him through the chest.