Hand of Argentum, Heart of Iron Pyrite
An original editorial by Peter
Most sources that I have seen agree that this series will see the end of the Marauders. We have lost Padfoot and Prongs; only Moony and Wormtail remain. That night, years ago, Wormtail essentially sealed the fate of the first two: he betrayed James to his death and had Sirius framed for it.
I believe Peter, the weakest of the four, the tag-along, is critical to the end of the remaining two. Of course, he has had charge of his own destiny, in a sense, but the control of Lupin's fate is the more interesting of the two.
Lupin has never had a great deal of power over his own life. As a werewolf, he is limited in what he can do, where he can work, who will have him for dinner. His friendships have been vital to him. He bends himself and his values backwards to keep these friendships whole. He does not tell Dumbledore about his Animagus pals; he couldn't stand a rebuke and the loss of respect from one of his few benefactors. He never keeps the marauders in line, even as a prefect. Needless to say, he has inferiority issues.
Peter, however, will do anything to protect himself and his interests. These two, among the Marauders, represent the greatest direct contrast. Where Lupin is brave and true to friendship, Peter is cowardly and deceptive. Both have had a sort of stigma upon them. Lupin openly lived as a werewolf, taking the financial and social blows inherent. Peter hid as a rat, hoping beyond hope that he would not be discovered. Therefore, it is fitting, I think, that the activity between the two will eventually cause both of their downfalls.
There are a few facts to consider. Peter now has a silver hand, an important clue Rowling leaves us for his purpose in regards to Lupin. Remus was reluctant to let him live, bowing to Harry's more direct right and the need to prove Sirius' innocence. When push comes to shove, he will take his chance to kill Wormtail. Peter, given the chance, would also wish to take his vengeance. He will not easily forget the eagerness of his old friends to finish him that night. Also, Lupin is an enemy of his master, any cause to raise his standing with the Dark Lord would certainly be worth taking. Peter owes Harry his life. He cannot refuse the magical contract.
Different theories have been tossed around as to how the debt will be repaid. Dumbledore did express his notion on Voldemort's reluctance to accept an indebted servant back into his graces. If Lupin or some other member of the Order doesn't kill him, certainly Voldemort will.
I believe his loathing for Lupin and the chance to relieve himself of his debt to Harry may be more intertwined than any other theory has suggested. Consider what will likely happen in HBP: a great battle will lose us some of our close characters, Harry will survive with the resulting emotional scarring, and the lines will clear for some of these in-between characters (Snape, Wormtail). Leaving Snape aside, I think our big moment for Wormtail has arrived.
With the location(s) of this battle unknown, there is room for speculation. We know the war has begun and Muggles will notice it. The Statutes of Secrecy will not prevent the Order from openly confronting Voldemort's followers. Granted the preceding, the battle to come may take place outside. Rowling has staged all of the climactic moments in either secluded places or indoors. Perhaps she may make this next one in a much more visible place.
If they fight outside, the moon may come out and Lupin would turn. In this state, assuming he has not had his potion (which in the state of war could happen), he will lose all control. With no Animagi to make him calm, Lupin will attack anyone and everyone, given a chance.
I contend, at this point, he may turn and attack Harry. Harry will have trouble making any counter; he will hesitate. As Lupin barrels down upon him, only one individual will have time to react. Peter will step in between them and paying his debt and appeasing his hatred for Lupin, kill Lupin with his silver hand.
This will leave Harry in shock; he lost a great friend and was saved at the same time. What emotion could he have for a man who has done this? Knowing Harry, bitterness would probably well up, but confusion would set in. He would lose the opportunity, while Wormtail is injured from the attack, to avenge his parents and Lupin.
But that doesn't mean Peter will get off so easily. His false sense of accomplishment and redemption will not please his Dark Lord. As we have seen in Order of the Phoenix, the Dark Lord would rather kill Harry outright. He has no reason to keep him alive. So, a servant that had the chance and blew it would displease him greatly. I believe Voldemort would finish him off afterwards. I imagine the scene paralleling that from Goblet of Fire, Wormtail begging for help as the Dark Lord looks on, unmoved.
In the end, Wormtail's play for personal advantage will be his undoing. In attempting to please both sides, he will anger everyone. And Lupin, doomed to the fated monthly transformation, will die completely out of control. As he lived, so he will die. Then, we will bid farewell to the last of that pleasant bunch who sat beneath the tree after an exam, enjoying the afternoon sun.