Jeff the Killer had escaped from a mental asylum. A marine team was sent in to eliminate him. They arrived in a helicopter and found the institution has been destroyed, and saw Jeff weakly pushing the destroyed walls and the remains of his injured body.
He looked up at the Sergeant and stood up, but collapsed as he died of heart failure. They took him to a fire pit and were going to burn his body when one of the bio gas containers opened. Jeff had a plan to kill everybody, all along. He faked his death. The Sergeant escaped on a chopper with another marine, while the others died.
Jeff can never be killed, so don't risk doing it.