To attempt to alter the constitution or laws in the 50 states to prevent something like this from "ever happening again" is VERY unlikely to work. If criminals feel that they gain more advantage, by assuring them that they will RARELY encounter any armed resistance, they will commit more crime, not less. Even though the media is all hyped up over this ONE event in CO, we must remember that far more than 12 die in a typical week in Chicago.
Do you REALLY think that criminals would NOT use guns if they were illegal? We KNOW that criminals still sell drugs even after they were made illegal. A national gun ban is NOT going to happen, we already have a national crime ban. Fortunately we must wait for crime to happen to make an arrest, you seem to want "preventive" arrests simply for having guns, exactly the same as the "war on drugs", but strangely we now IGNORE most of those that simply have or sell drugs, and go after the "big fish". It is not more laws that we need but very swift and sure JUSTICE for those that break them.