Once Stu "killed Billy" and began chasing Sidney around the house Billy donned the costume to cut her off. Stu would have killed Randy had he not heard Sid screaming for help. They both followed her out to the van where Billy kills Kenny and gets stuck as Sidney runs off. Stu helps him get out of the van and they hoist Kenny on top of it.
Sidney continues to wander around looking for anyone while Billy and Stu return to the house. Billy stabs Dewey and deals with Sid at the cruiser while Stu is around back with Randy who just found Tatum's body. Once Billy dips down out of view of Sid at the window he gets back in the house and upstairs as Randy and Stu come around the corner.