Just realized: Remember Elan and Nale's conversation about "The villain is never really dead unless you see the body (and half the time not even then)"?
Roy just proved Elan right! The hero always thinks the villain is dead. Roy (and Belkar) just walked away from Thog, despite the fact that the barbarian probably had more than enough HP to survive a ceiling falling on him. Considering that even a huge object falling does only 6d6 damage, an average of just 21, I think it's rather unlikely Thog was finished off there. More likely he's just stuck under the rubble and can't force his way out.