It means that it takes people to kill people with that gun. A gun can't just hop out the holster and automatically shoot it's victim.
Extended Version of "ROMEO MUST DIE"
DMX: Guns don't kill people, People kill people.
Po: You stupid as fuck, nigga.
DMX: Shut the fuck up Chinko before I chunk yo ass up, bitch. This gun can't jump out my God Damn arms and shoot your bitch ass up.
Po and his Crew: Oh.
DMX: (Sucks Teeth and Shakes Head) Stupid ass niggas.