Why do people kill?

In recent time I was watching a lot of TV shows about killers and it led me to question myself, why people kill other people? This topic is so large that even I have problems to describe all my thoughts about it. Is it so frequent just nowadays? No, of course not, people were killing each other in the past as well. Reasons and tools used for killing might differ, but the point is the same; mankind killed in the past, mankind kills nowadays and mankind will kill in the future as well. For whatever reasons with whatever tools.

But what's behind this? Why can't person control himself when he is aware of predictable consequences of his behavior (like jail)? Of course, I realize that some people might have some reason to kill another human being; when they are endangered it can produce agression and lead to killing someone; or when they see some real injustice I can kind of understand their motives to kill someone in order to take revenge. But still, not all killers are psychopaths or sociopaths with real personality disorders!

Sometimes even people considered normal, with "normal"psyché kill. I personally think that except serial killers who are mainly sadists and like to see their preys suffer majority of people kill because of lack of conscience and moral values. Yes. I don't blame lack of money or some affairs responsible for this. Every person can cope with these things, feelings, whatever even without killing, but some people just don't! And I blame poor morals and lack of self-control for this. First thing what led me to think this was that every person can experience emotional trauma, distress, can be betrayed, cheated on, whatever, but they won't go to kill someone. Second thing what led me to blame poor morals (which are not transparent of course, that's why people often blame affairs, betrayal, lack of money and etc.) is that even majority of people who have alexithymia won't kill people. Why? According to many of them they don't really care about people, don't like them nor miss them, but they won't kill them. They don't need emotional attachment to know this. Because they still have conscience and self - control! They still have morals, cognitively created construct in their heads which won't allow them to do that. So my question is, can we really blame emotions responsible for this from any point of view?

Poor morals must mean that the society failed. Including parents and the closest social enviroment and the widest macroenviroment as well. You might noticed, there's something missing in the schools. Just forcing kids to learn some knowledge isn't enough. They need to think, not just memorize. People forgot to think. Also, ethic educations sucks too. Knowledge without having real morals and self-control means NOTHING. Even killer can be intelligent. Plus, I would suggest completely different way of learning with highlighting the way of searching for informations and comparing them and then explaining instead of dull memorizing, but that's off of this topic.