That crumpled heap in the background covered with a sheet is Bart Bass's corpse. He DID fall off the building, and presumably, he can't fake his death this time because his brains are busy making a real estate transaction of their own with the sidewalk. Chuck and Blair are dumb. Nobody knew they were on the roof with Bart, yet instead of putting themselves back at the party and looking innocent, they run away. And because Chuck told everyone Bart tried to kill him, he's going to be Suspect #1 here, and RUNNING MAKES YOU LOOK GUILTY YOU FOOLS. YOU HAVE AIRTIGHT ALIBIS. NOBODY SAW YOU. Sigh.