Sure, people kill people with guns, guns don't do it by themselves. That's a pretty shallow observation, though.
The point is that guns enable people to kill much more easily and quickly. If guns didn't exist or weren't allowed, there would still be criminally malicious people and murderers. But it's much harder to kill 20 or 50 people at a time with a knife. Murders would still happen--mass murders would be far, far, far rarer. Not because people would suddenly stop wanting to mass murder, but because mass murder would be much, much, much harder.
Let's stop with "guns don't kill people, people kill people." It's trite and meaningless when it comes to the discussion of the effect of unregulated firearms on society. "Society" already denotes that people are the actors. The question is whether we want to allow guns to be an accessible tool for those people.