I think point of trying to ban the guns that fire off dozens of rounds in a matter of seconds is that while school shootings and massacres can still happen, at least the carnage won't be as high as previously. I can understand some of the counter arguments, yes these guns are used very rarely, but I don't think it means we shouldn't do anything. For example, the 9/11 attacks still impact my trips to the airport, I get patted down and have to spend a good chunk of time with airport security - even though it's was an extremely rare occurence, I think most americans are happier that are planes are safer, even if the potential for a hijacking still exists (no matter how small it may be). Same thing here.
The other argument is that gun collectors enjoy having this type of stuff, I don't think that's a really good reason though. The other reason, to help people fight against a tyrannical government, I think that reason is incredibly out dated though, if you have an issue with city hall or something, going there with a gun will likely end in you being killed.
As for other types of murders, there are methods of prevention, obviously they don't always work, things such as gang education in school, programs that help the unemployed find jobs, ect. There will never be one answer to these types of problems and you'll never fully eradicate the problem to begin with, but I do think there is a lot we can do to decrease this type of violence.