How does a creature retreat with 1 (or 0) or fewer hit points?
I've ran into this twice now, don't remember where the first one was but the second was in We Be Goblins, Too!
In We Be Goblins Too! the creature Piggy in it's Morale section says it "...attempts to flee back to the Munchmeat farm if reduced to 0 or fewer hit points."
Granted at 0 Hp it can still move but anything more than a move action, like double move, would cause it to take 1 hp damage. And last I checked anything, except undead, falls unconcious at -1hp.
well the short answer is it might just retreat and take the AoO.
If at 1 hp you can do a withdraw action which is a full round action, and you can move up to double your speed, and don't provoke AoO from the first square you leave. Now unless you are really fast, or your foe merciful, a single opponent fleeing from a party is probably toast.
If the creature had die hard feat they are staggered but can still move, os they can take the 1 hp loss and provoke an AoO while retreating no so long they aren't at -con in damage.