You can't assume the Colo shooter would've been able to get a gun if they were illegal. From what's been reported, he was a medical school drop-out who worked at a bar, he wasn't a military guy. Can the average person go out and get a rocket launcher? No, because they're illegal for normal people to have and therefore, normal people don't know where to get them.
The argument that if guns are outlawed, only outlaws will have guns is straight up retarded. Yes, if guns are outlawed, only very certain outlaws will have guns, i.e. a miniscule fraction of the number of people who have and use guns today. You think most of those bonehead gun nuts would have any clue how to get a gun if they were outlawed? Give me a break. Gun violence would absolutely plummet if they were outlawed.