"'We ... we are alone, aren't we?' Narcissa asked quietly.----HBP, p. 28-29: Spinner's End
'Yes, of course. Well, Wormtail's here, but we're not counting vermin, are we?'
He pointed his wand at the wall of books behind him and, with a bang, a hidden door flew open, revealing a narrow staircase upon which a small man stood frozen.
'As you have clearly realised, Wormtail, we have guests,' said Snape lazily.
The man crept hunchbacked down the last few steps and moved into the room. He had small, watery eyes, a pointed nose and wore an unpleasant simper. His left hand was caressing his right which looked as though it were encased in a bright silver glove."
At the beginning of Half-Blood Prince we learn that Peter Pettigrew is living with Snape, and that Voldemort sent him there in order to help Snape. Prior to HBP we hadn't seen or heard of much interaction between these two characters. We can assume that Snape lumped Pettigrew together in with his list of Gryffindors He'd Like to Hex, along with James, Sirius and possibly Remus. I doubt Wormtail went out of his way to antagonize or bully Snape, but from what we saw in the Pensieve scene in HBP, it's safe to assume he did nothing to stop it, but instead enjoyed and was entertained by it. Snape no doubt has a low opinion of him, and while Wormtail may still dislike Snape for being "Snivellus", Snape is a threatening presence in Wormtail's life in HBP, and not someone he's equal or dominant with.
It's quite strange to think that these two people, with their Marauders-versus-Snape history as teenagers, were suddenly thrown together as adults and Death Eaters. It doesn't seem like there's any way that Snape would ever respect Wormtail, and thus will never treat him well. There's no way that Wormtail could ever like Snape, partly because of their past, and partly because of the way Snape treats Wormtail in HBP.
"'Narcissa!' he said, in a squeaky voice, 'and Bellatrix! How charming--'---- HBP, p. 29: Spinner's End
'Wormtail will get us drinks, if you'd like them,' said Snape. 'And then he will return to his bedroom.'
Wormtail winced as though Snape had thrown something at him.
'I am not your servant!' he squeaked, avoiding Snape's eye.
'Really? I was under the impression that the Dark Lord placed you here to assist me.'
'To assist, yes -- but not to make you drinks and -- and clean your house!'
'I had no idea, Wormtail, that you were craving more dangerous assignments,' said Snape silkily. 'This can be easily arranged: I shall speak to the Dark Lord--'
'I can speak to him myself if I want to!'
'Of course you can,' said Snape, sneering. 'But in the meantime, bring us drinks. Some of the elf-made wine will do.'"
At the same time, though, I can't help but see similarities between them. Both were spies for opposite sides during the first war. Wormtail, a member of the Order of the Phoenix, began spying for Voldemort approximately a year before James and Lily were killed. Snape overheard the Prophecy some time before Harry was born and began to spy for Dumbledore at some point afterward. Then, at the beginning of HBP, they're forced to live together. Wormtail is now openly a Death Eater and Snape is still acting as a spy (for which side? I'm not going to get into that question here too much) and openly a member of the Order of the Phoenix.
During the first war they both had to answer to the same two people: Dumbledore and Voldemort. They have experienced how both operate, and how both treat their followers (although I don't know if I would use the term 'follower' for member of the Order). I can't help but wonder how much Voldemort and Dumbledore knew about the situation in HBP. Does Voldemort know about their history at Hogwarts? Did Snape tell Dumbledore that Wormtail was sent to live with him?
If Voldemort did put them together knowing what their relationship was at school, I can't help but wonder if he had a particular motive behind it. The obvious motivation is the fact that Wormtail is easily bullied, and Voldemort wanted to give Snape the chance to bully him. But there is no specific perk for Voldemort in this situation, other than giving Snape a chance to be cruel, and generally making life for Wormtail unpleasant.
If Dumbledore knew of the situation, no doubt he would realize that Snape would use the opportunity to make life unpleasant for Wormtail. This may simply be because Snape's strong personality and Wormtail's weak one makes it easy, and Snape enjoys it, but perhaps Snape sees it as a convenient opportunity to take revenge against a Marauder.
In the most basic terms, Snape and Wormtail are on the same side in HBP, yet even in the small glimpse we see of everyone's favourite rat-boy and his interaction with Snape, they are at odds. If they still see each other as a greasy, mean, evil Slytherin, and a stupid, James-worshipping Gryffindor who loved to see Snape get harassed, they are not on the same side.
"Wormtail hesitated for a moment, looking as though he might argue, but then turned and headed through a second hidden door. They heard banging, and a clinking of glasses. Within seconds he was back, bearing a dusty bottle and three glasses upon a tray. He dropped these on the rickety table and scurried from their presence, slamming the book-covered door behind him."---- HBP, p. 29: Spinner's End.
Snape's ill-treatment of Wormtail may be deserved, but regardless, no one likes to be treated as subservient. Wormtail especially must resent being treated like a slave by a fellow Death Eater, given that he literally cut off his right hand for Voldemort.
Oddly enough, I've always thought that feeling unappreciated or unvalued must have been a contributing factor towards Wormtail betraying the Order in the first place. He's never described in a flattering way in terms of his abilities or personality, but he was still a part of the group.
"'You don't understand! whined Pettigrew. 'He would have killed me, Sirius!'- PoA p. 275: The Servant of Lord Voldemort
'THEN YOU SHOULD HAVE DIED!' roared Black. 'DIED RATHER THAN BETRAY YOUR FRIENDS, AS WE WOULD HAVE DONE FOR YOU!'"
Clearly, the other Marauders were loyal to Peter, even if they did make fun of him. Sirius and James's dedication to the protection of every Marauder may have been equal, but they did not have equal amounts of respect or esteem for Peter as they did for each other.
"'Put that away, will you,' said Sirius finally, as James made a fine catch and Wormtail let out a cheer, 'before Wormtail wets himself with excitement.'--- OotP, p. 568: Snape's Worst Memory
Wormtail turned slightly pink, but James grinned.
'If it bothers you,' he said, stuffing the Snitch back in his pocket. Harry had the distinct impression that Sirius was the only one for whom James would have stopped showing off."
Judging by the reactions of the characters, the impression I am given by this scene is that poking fun at Wormtail in this manner was not uncommon. Then there's the fact that Wormtail needed help with becoming an animagus, and possibly in other areas as well. One or two incidents of being made fun of may be forgiven and forgotten, and gratefulness for help may have outweighed feeling inferior, but if Wormtail was repeatedly treated this way and did nothing to stand up for himself, and constantly relied on others for success, surely he must have resented it and caused damage to his self-esteem.
Even as young adults in the Order, one can easily picture James, Sirius and Lily going off and doing brave and dangerous things like fighting Death Eaters and facing Voldemort. But Wormtail? He doesn't strike you as the type of person to be assigned such tasks or seek them out. Instead he likely was given simple jobs to do, all the while watching his friends do more important things. Then somehow he comes in contact with Voldemort (or possibly a Death Eater, but given how little known it was that Wormtail was alive during the years Sirius was in Azkaban, it seems more likely that Wormtail was in direct contact with Voldemort, in order to keep his position as spy a secret), whose methods of recruitment involve promising power and glory, along with threatening death and torture if one does not agree. As someone who is obviously afraid of danger or punishment and who has never experienced power or glory, Wormtail would have found this attractive, even if it did mean betraying his friends. One can almost hear Voldemort saying: "Are they really your friends? Look at how they treat you...", etc, and using the inequality between the Marauders to his advantage in fueling Wormtail's motivation to betray them.
After having faked his death, lived as a rat for 13 years, finding what was left of Voldemort, nursing him to life and mutilating himself to help restore Voldemort to his body, we find Peter Pettigrew, shacked up with Severus Snape, who generally acts like the eloquent asshole that he is.
If Voldemort is aware of their history, he's not really thinking about what kind of pressure it may place on Wormtail, to be back in a situation where he is unvalued and made to feel second-rate. If Dumbledore was aware of the situation, I can picture him thinking, with a twinkle in his eye, that a situation like that may play out advantageously in the end, for Harry and/or the Order.
Being bullied by Snape cannot help but bring back Wormtail's memories of being side-by-side with his fellow Marauders, being protected by them, and while they didn't always treat him well, as least they were loyal to him. Wormtail may feel nostalgic for those times, and may start to reflect on the differences between working for Voldemort (who operates using fear and vague promises of power) and working for Dumbledore (who operates using trust, friendship and "doing the right thing"). This is not to suggest that Wormtail should gladly accept being made fun of and being underestimated (because let's face it, he blew up a street [killing 12 people], faked his death and framed someone else for it: he is magically powerful and he is clever), because much of his own suffering, IMO, has been brought on by his inability to stand up for himself, and by always relying on others for leadership and protection.
People have often asked why a person as cowardly as Wormtail was in Gryffindor, and believe that surely he'll do something to prove his Gryffindorness by the end of book seven. I believe that the main thing he can do to redeem himself isn't going to be a sacrifice (although I'm sure his actions will in some way help Harry), as he's already sacrificed his hand for Voldemort. He could have easily just dumped Voldy in the cauldron in the graveyard, without chopping off his hand, and run the hell away, but instead he stayed and made the sacrifice. He's also sacrificed his life as a free man, and chose to live as a rat for years on end. No, if there is one thing Wormtail can do that would be truly brave, that would be to stand up to those who are cruel to him, to take a firm stand somewhere --anywhere-- and believe in himself enough to stick to his guns and fight for it.
Due to his life debt to Harry, and because living with Snape may have made him nostalgic for unity with the Marauders, it makes sense that when Wormtail finally grows a spine, he will do something that benefits Harry, Remus and/or the Order and the fight against Voldemort.
(All quotations are from Bloomsbury/Raincoast editions of the books.)