It would be impossible to clean up the streets of guns.
If someone wants a gun to commit a crime to this extent, they will find one, one way or the other.
To the smart ass, how about a more strict background check for a man who was clearly described as "troubling"? If he truly wanted a gun, I'm not saying he couldn't get one. But with more strict gun laws, this would unquestionably reduce the amount of pointless, insane incidents like this. If that's too hard to comprehend, which it probably is for most people on here, then I can't help ya.