Finishing the School Shooter's Work

Okay, listen up, you simpleton. You created this garbage, so let me break it down for you, since you are clearly not the sharpest tool in the shed. The joke is about a school shooter getting killed, and everyone is cheering because they're glad he's gone. Then, someone else comes along and says they'll finish what the shooter started, implying they're going to continue the shooting. Get it? It's dark humor because it makes light of a tragic event.