The Redbox Killers
The Redbox Killers are a gang seen in the Season Sixteen episode, "A Nightmare on Face Time".
Background
The gang believes the Redbox DVD vending machines have money inside of them, and hold up a Kum & Go convenience store to steal the money. The boys see them while trick or treating, and decide to intervene. Noticing the store clerk was murdered, Kenny, Kyle, and Cartman run away leaving Stan on the iPad in the killer's hands.
The gang also believed iPad Stan was a real person, and tied him up to a chair. They tried to torture him with a knife and threw him in a field.
Later, at the Park County Community Center Monster Mash costume party, the gang arrived to find the other boys who saw them at the convenience store. Sergeant Yates arrested the gang by disguising Officer Peterson and Stan so they could point them out in the crowd.