What needs to be done?
A lot, in my opinion. You've got lots of potential "causes" for gun violence. Ultimately it's that person's individual choice to act with violence, but people's choices can be affected by lots of things.
For one, stupidity. Not sure what can be done there, other than to arrest stupid people when they make stupidly illegal decisions, and hope it gets through to them. In that regard, there's a lot of reform that needs to be done to certain prison and jail systems, but that's a whole different topic. But in regard to trying to prevent stupid people from being stupid? Not gonna happen.
Mental diseases, issues, conditions, problems, whatever you want to call them...they should be addressed. I think we need to be careful how we identify mental diseases, because too many people are being medicated for no good reason. Example: hyper active children being given Ritalin. While there are children who may medically need it, there is rampant neglect or outright abuse in the system that's tolerated. But overall, we need a better mental healthcare system than what we have. Considering our financial constraints, I don't know how much can be done by the government.
DHR. Make it a better system so as to identify and properly deal with problem homes. If you're going to have an agency that is supposed to protect children and prevent them from growing up in abusive or harmful environments that could affect their psyche, then at least do it right. Advocate that potential parents can adopt from America more easily, rather than forcing them to go to China and Africa to get children. That way, when we have to take children away from parents, we don't deter adoptive parents and can deal with the children more efficiently.
I think you're always going to have completely sane people with no history of mental illness, who break suddenly due to either one traumatic event, or a build up of anxiety over time that eventually bursts. I don't know if that's happened here; I haven't cared to read on the killer's history. But in general, it can always happen no matter how hard you try to prevent it.
But if we're talking about what
should be done? Pie in the sky hopes without financial constraints or issues with expanding the government's control? Then yeah, I think the above issues need to be addressed better than they are.