A few relatives of mine have been murdered in recent years (last 15), I won't go into the details because I am sure my family wouldn't want me to talk about the specifics here.
Would gun ownership have helped in their cases one way or another? It is really hard, imho, to say that gun ownership would have helped them, just as it is hard for me to say with any level of accuracy that gun control would have hindered their murderers (who in one of these incidents, were also relatives).
There are other nations with similar guns per capital stats, without the gun related violence that we have- even Moore acknowledged this. If the problem is specific to a society that is merely violence-happy, gun control really is only going to be a bandaid on the bigger issue, just as some countries have shown an increase in knife/blade related violence after engaging in stricter gun control regulations.
I'm not going to pretend to know the answer to violence & crime statistics, but I am not convinced that gun control will end violent crime anymore than the idea of prohibition ending alcoholism.
I do think a good start would be a half way decent public education (not schooling) system. Our schooling system is one that encourages its disillusioned students to drop out, in some American cities this drop out rate can exceed 75%- and that only breeds violent crime.