The gun does not pull it's own trigger, it has to be the person to get that done. Notice I didn't make mention of why the trigger is pulled, just that it is.
Gun control advocates like to say "Well, the intent must be evil."
Until you know the intentions of the person with the gun, you can't say whether or not their motive is to kill others in cold blood or something that's not evil. To do so would be to presuppose guilt based on the person in possession of an inanimate object that doesn't itself have motivations nor intentions.