See this I think is part of the problem. We're not talking about a Die Hard film here, we're talking about real life. There are no bad guys or good guys, just guys going through their lives and doing the things that make sense to them. The guy who killed someone in a cinema because he was texting and throwing popcorn was an ex police officer - one of the good guys. Human beings don't fall into such neat and easy categories; you can't say that someone, anyone, will categorically never make a mistake with their gun and end up killing someone. The difference between bad and good is simply a matter of clarity of judgement, and humans have proven countless times that they make mistakes. And of course, pretty much every gun which ends up in the hands of a de facto bad guy, will be in circulation in the first place because it was bought legitimately by a 'good guy'. If you accept that guns are a necessary part of society, then you have to accept that it's inevitable that infrastructure which causes guns to fall into the hands of people who would use them to attack society.
Don't get me wrong, 'm generally in favour of gun ownership, but I don't like simplistic black and white arguments about good and evil, because they're grounded in fiction, and you're talking about decisions which take place in real life. Of course when you ask the question whether or not guns should be legal, you have to look at where the country is currently. In the US, if you scrapped the second amendment and abolished all legal gun ownership, it would be an absolute catastrophe - there are already millions of guns in circulation so all you would do is take them out of the hands of honest people, whereas the massive number which are in circulation illegally would remain. If you were designing a new country however, sim-city style, and deciding whether or not guns should be legal, I would probably sit on the side of non-ownership.