And then, in that particular chain of event, crime IS averted, I agree with you. But that's only part of the problem. In either cases, crime is not averted. So in that case, crime is really not averted, it is fatal. Bomb killings can be tracked a little bit easier.
Guns add anonymity.
If the gun is ever recovered, of course it can be identified by bullet marks but with the quantity of guns available right now, there aren't a lot of chances. The only case I could see that can be potentially useful is if someone break in your house while you're asleep. Which doesn't mean you can't call 911 and leave the phone there, take some kind of sharp or heavy object and prepare to give a big hit if someone goes near your room's door. Of course, it's not as effective as a gun but anyways. If goons were intelligent enough, they would do EM bombs instead of usual blast bombs.