Demon Lord Belial. I kill some monsters and watch the sniper and then Death gets all "my name... is NEO" what with the bullet time evasion trick. Then I kill the rest of the demons and.. all the doors stay red.
I can't go back, as in exit the area from the previous door and then return in hopes of "resetting" the area or whatever. I can't move forward, because the proper door on the left won't open.