If you want to get technical about it, guns don't kill anyone. The force and blood loss caused by bullets fired from guns cause death. I guess it is about what definition of "cause" you want to argue about, since there are various types.

What is the point of a firearm, anyway? Well, they are relative easy to operate, require little skill to be effective at close range, and are a force equalizer. That means that a little old lady can deal out just as much (or more) force to an attacker than the attacker can to her, regardless of his size and strength.

Get rid of guns and we're back to being at the mercy of the biggest, strongest, and fastest evil-doers out there. Ever seen what a knife attack looks like? Go ahead and google some pictures; it's pretty gross.

Firearms do serve a purpose: they exist to deal potentially life-ending force to others. Is this bad in and of itself? No. The person who uses it to save their life or the lives of others against an assault is using it for good. The statement "guns don't kill people; people kill people" may not sound good to you, but it is meant to convey a simple truth: inanimate objects are neither morally good or bad by themselves, but can be used by humans for both good and bad ends.

The function of a gun is irrelevant to this discussion, though, if the discussion is about lives saved and lost. If you want to talk about that by itself, then we can start talking about hammers, pools, cars, fists, and feet. If you want to go back to the "purpose" of a gun compared to those other things, then you are getting into shaky ground, since all things have positives and negatives to them.