If not guns, people would use knives, baseball bats, truncheons, fists, and so on.
That's true for most of your run of the mill crimes of passion, etc.. But just to play devil's advocate here; without guns most of these mass murders wouldn't have occurred. How far do you think the Aurora shooter would have gotten with a knife?
Don't get me wrong I fully believe in our right to own weapons, and I don't think that laws should punish the many for the sins of a few. We just need to make sure that our arguments are based on facts and reality. After all, we often claim that that is what separates us from the other side.