Where does violence come from?

I think violence comes from two main sources: the mistreatment of mental illness and the environment a kid is brought up in.One of the biggest examples for mental illness is if you look at it, most of the mass shootings that happen in the U.S are by people that have so some sort of mental illness whether its something like depression or schizophrenia, and they don't get the proper help they need. Another reason is, I feel like when a child is brought up in a bad environment, they can either turn out really good, or they can turn out really bad. Some kids want change, and they strive to become everything their parents were not, while there are other kids who just fall behind, and you cant really blame them for that.