Complex sentence: The Outsider then threatened J'onn to tell him about any future assassins, when J'onn refuses, the Outsider closed the teleport cutting J'onn in half killing him.
Split into simpler sentences: The Outsider then threatens J'onn to tell him about any future assassins. When J'onn refuses, the Outsider closes the teleport, cutting J'onn in half and killing him.