Tull was a necessary evil for Roland! He couldn't leave anything to chance... the trap was set and if he left it could have come from behind as he entered the desert with nowhere to hide... I did find it highly sexualy charged, with his sexual encounter with the bar keep (forgotten her name right now) and the preacher woman! But the demon the MiB left behind is more my interest. Would it have been a Mordred type demon or something more like the sex demon's we got to know of in the circles??? As we know Roland was obsessed with the tower and for good reason, the whole of (his) mankind depended on it...