You're not wrong for wanting to stop these murderers from getting guns, and of course we all want to prevent these kinds of senseless tragedies. The only point I wanted to make is that, unfortunately, gun control policies (at least any that I've ever heard suggested and any that I can rationally concieve of) would be completely ineffective in doing so.
As for whether gun control can be of any benefit whatsoever, I think there are other factors involved here (i.e. freedom vs security, etc) that shape my thinking in a way that I will reach different conclusions than you might because we have different/diverse moral priorities/political philosophies, and that's fine.
I definitely don't have any overarching solutions to the problem of seemingly random occurrences of mass murder. But neither does anyone else that I can see. Sometimes life just sucks and while we'd like to believe that we can fix it by waving a magic wand or some new arbitrary rule, I don't think thats the case.