Why do people kill?

I'm not sure why people kill. there really isn't any explanation or justification for it. I think it all stems from a person being mentally ill in the head and they think by taking another human's life, they can get satisfaction or gratification. I have never spoken to a musrderer. I think people that take the lives of another human being should also have their lives taken if it was not done in self defense. There is no excuse for someone to kill someone else unless they are protecting themselves.

Out of the blue, some people kills because they think they could satisfy themselves from their self-interests, desires and expecting to be more happy later. That's without realizing the nightmare they may have after. Those are the kind of people who are selfish and evil. They don't give value to life and they dont even fear God. And they dont deserve to live a life peacefully.