Earlier quoted context omitted.
The question is more why do we treat assault and homicide where a firearm is involved differently from other instances of assault and homicide? Dead is dead, no matter the manner in which it happened.
Why do pro-gun agenda people always say that violence would continue to exist without guns? Do they think people would replace guns with knives? What makes them think that people wouldn't just give up?
People in general are pretty clever. If you take away someone's means to do something without addressing what motivated them to do that thing, they're likely going to figure out a way to do it anyway. (See: censorship, DRM, drug prohibition)