I did not grow up with guns around, never owned a gun and rarely fired one (th last time being during the mid-1970's). So I don't have a vested interest in this debate. But, I do have some observations.
As with so much of the violent and/or street crime in U.S., it seems to me that most gun violence is perpetrated by a small percentage of Americans. It seems to revolve around certain locales and certain activities. I can't tell you how many times people have told me, "If you aren't involved in the drug trade and if you don't hang out in places that most everyone know are bad news, you have little to worry about in terms of violent crimes." And, I think that is true. There is an element in American society, and turf they dominate, that produces most of the violent crime, including gun deaths.
I've not been to Europe, so maybe I am incorrect in thinking that European countries are different culturally/socially from the United States in a way that affects the frequency of violent crimes (beyond just gun laws). Certainly, you can ban things in U.S. and people who disregard laws will get those things anyway. If their interest is to defend their turf, their drug revenues, and ward off competitors or people who don't pay, they will get guns to enforce these things. As the old saying goes, "The only people who will be disarmed by gun laws are law-abiding people. The criminals will still get guns." I am wondering if Europe is different, in that they have tough gun laws and bad guys still tend not to get guns . . the black market for guns is not such a factor there.
If my observations are correct, that still doesn't tell us how to change the violence-prone people in U.S. so that they won't resort to lethal force to obtain their objectives. Regarding the Colorado case, radio commentators have made the point that someone as smart as James Holmes would figure out a way to obtain guns, despite any laws. Or, that he could develop explosive devices to kill and maim people with.