How do people act aggressively?

Probably a very frequent question. But how do people act with cruelty towards others?

I can understand being mad at someone, maybe even swearing at them or thinking bad thoughts about them.

But I dont understand how people can be pushed towards violence or unmitigated aggression towards others. Is it something with people's background that enables them to act with aggression? Or are some people just born brutal?

I can understand aggression in self defense (e.g. if someone is swearing at me and getting in my face I can shove them away), but if nobody was the aggressor there would be no need for self defense.

What do you think?