But why did he close the curtains ? ... and why did he bother closing the front door behind him ?
Did Kate actually know if the curtains were fully closed? Or maybe partially open but still whooshed fully open with a gust of wind. Very gusty that night.
Imo Icabod, he closed the curtains, partially at least, after the deed was done. He left the window open to help clear any ?chloroform fumes, but pulled the curtains to a little. In an emergency he could still escape thru the cheap curtains if the window was open.. I think at one stage they were fully open to provide natural moonlight and a little light from street lamps that managed to get thru the trees, to the room. Better to use that light rather than use artificial light within the room.
I think he closed the front door cos the key was left in the lock and it was easy to pull it to using that. That way there wouldn't be any finger prints on the front door, just on the key possibly
And he was taking the key with him.
I think the main purpose of closing the front door and leaving the window and shutters open was to give the impression that Madeleine had left under her own steam, to give them getaway time. It was also important to take the investigators eye off the front door, cos that would point to someone attached to OC, who had accerss to the keys.