Complex sentence: When a zombie bursts into the room however, Peter then changes his mind and turns the gun on the zombie and fights his way up to the roof, where he joins Francine, the two then fly away in the partially fueled helicopter to an uncertain future.
Split into simpler sentences: When a zombie bursts into the room, however, Peter changes his mind and fights his way up to the roof, where he joins Francine. The two then fly away in the partially fueled helicopter to an uncertain future.