It's really easy to say 'if there were no guns', but it didn't keep it from happening in Norway. The reality is that things like this happen. Japan has no guns, and almost no gun deaths, although some people claim that they simply don't acknowledge shootings. That would fit the Japanese way of doing things.
The reason that these shooting have steadily increased since 1981 is fairly obvious. Ronald Reagan closed all the insane asylums and didn't create the halfway houses he promised. Republican crony developers made billions off of the real estate that was held by these institutions. Often created by 'land grants', many had a square mile of what had become over a hundred plus years, very valuable land.
Ordinary day to day gun violence is largely a product of the inequality that Reagan and his disciples have brought us.
We need to take seriously the treatment of the mentally ill, and the confinement of those who need it. Electronic monitoring might even be an option, as long as no 'for profit' corporation was involved.
It would be a good idea to force the media to stop glorifying these killings. When they stopped covering people jumping from buildings, that ceased to be a big problem. It didn't actually reduce the suicide rate, but it was safer on the sidewalk. The same would apply here.
Gun control? We'll have that when Blackwater goons going door to door seizing guns like they did under Bush in New Orleans.