Before the guns, education, religion and anything else, people fought each other. So the main reason is our human nature.

Overpopulation got 2. Probably Thanos and Ebony Maw!!

Maybe all the above in small ways and of course varying based on where you are and what your background is. There's not one thing you can point at. I think the big ones are mental health and upbringing.

Well seeing as overpopulation is a myth.

There's no single contributing factor to the violence itself. or rather the violence is both too complicated and overly simple.

what we can do is circumvent the violence with laws. Humans will find a way to kill each other regardless. Guns just make it convenient. That doesn't mean we shouldn't do a better job of making things safe. Nor does that mean that ease at which these weapons are available is completely ok. There are options that we have.

charlie manson wrote all about his mom and all the different men she would bring home.

just one case, i know.

Ban violence in films, tv, and other media.

Environmental then, society.

Too broad a question. Each region has a different reason.