What is to blame for all the shootings/violence?

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.