So the only two true life debts we know of are

1) James’s saving Snape’s life when Sirius sent him into the Shrieking Shack to face Werewolf Lupin, and

2) Harry’s saving Wormtail’s life in the second Shrieking Shack episode when Lupin and Sirius were about to kill him

So I’m thinking a life debt is incurred not by saving someone from death in any and all circumstances (if that were true, then the Healers at St. Mungo are owed a lot of life debts), but more that it’s saving the life of your enemy, and maybe more specifically, taking action to save your enemy’s life when another person is attempting to kill him, or even more specifically, taking action to save your enemy’s life when your friend is trying to kill him.

The more I think of it, the more convinced I am that it’s the last situation that incurs a life debt: taking action to save your enemy’s life when your friend is trying to kill him.

So in that sense, if Snape is *good* because he’s on Dumbledore’s side, then there was no life debt incurred when Snape saved Dumbledore from the curse on the ring because he was saving his friend’s life. Ditto for Snape’s saving Harry when Quirrellmort was trying to kill him. Those are not actions that incur a life debt.

Draco’s situation is murkier because Dumbledore prevented Draco from becoming a killer, but he didn’t really intervene to save Draco from death. It remains to be seen if Draco is going to be punished for failing to kill Dumbledore when he had the chance.

James definitely saved Snape’s life when Sirius (James’s friend and mutual enemy of Snape) tricked him into the Shrieking Shack to face werewolf Lupin.

Harry definitely saved Wormtail’s life when he prevented Lupin and Sirius (Harry’s friends and mutual enemies of Wormtail) from killing Wormtail at the end of PA.

I think the conditions of a life debt are that specific (saving your enemy’s life when your mutual friends are about to kill him). And it works thematically because to repay the life debt, the saved person must do something similar in repayment–go against his friends in order to save his enemy.

In order to repay his life debt to James, Snape had to expose his own position as a Death Eater and betray Voldemort in order to tell Dumbledore that Voldemort had interpreted the prophesy to apply to Harry Potter and that Voldemort was planning to kill the Potter family, including James.

In order to repay his life debt to Harry, I expect Wormtail is going to have to betray Voldemort, if not personally, then at least betray Voldemort’s interests, in order to save Harry’s life. That, as you know, is how I see the secret passageways into Hogwarts coming into play–assuming I’m correct that Voldemort doesn’t know them all but Wormtail and Harry do.

We can see that owing a life debt stirs the conscience because Wormtail suggested Babymort use a wizard other than Harry for his rebirthing. So I’m intrigued by the idea that repaying a life debt triggers a real conversion toward good because when Snape returned to Dumbledore to tell him the Potters were marked for death, he wasn’t simply passing on information (something he could have done by owl), he had, I believe, truly repented of his actions as a Death Eater. So if that’s the case, then when Wormtail repays the life debt he owes Harry, he should also experience a true metanoia.