Warning: spoiler about P2: King of the Trollhaunt Warrens. The following explains why I want rules on killing yourself, but isn't critical to the question (with the edit), so don't read it if you will ever run this adventure!
In the last battle of the module, each time the Troll King dies, he reforms next to a cauldron in one corner of the room at the top of the next round. He then has to retrieve his magical eye from an alcove on the other side of the room. The party has to kill the King, grab the eye before he can get it, and drop it in the cauldron to finally kill him. Our party managed to get the eye near the cauldron while the king was locked down by another party member. The GM decided that the King would try to kill himself so he could reform near the cauldron and stop the player from dropping it in.
So, are there rules that govern how to kill yourself?
EDIT: If you didn't read the spoiler, I am looking for ways to kill yourself in the middle of combat. With other enemies that, assuming they know your motives, may want to prevent you from killing yourself.
A few options my group considered:
Can you attack yourself with a melee weapon and automatically Coup de Grace yourself, dealing critical damage?
Can you forgo your ability-score based defenses (basically like becoming flat-footed in 3.5), allowing your enemies to hit you more easily?
Can you voluntarily impose the Helpless condition on yourself by not defending against attacks?
EDIT2: I realize that Rule Zero is a perfectly valid answer to this question, but my group (and my GM) likes to stick to the rulebooks, or at least pull from them if we are inventing a houserule. So with that said, let me slightly rephrase the question:
What's the best way to kill yourself, within the normal rules for combat?
Note: I'm assuming melee attacks only here, no fancy spell-based stuff like walking into your own wall of fire.