Peter Pettigrew was in love with Regulus Black.
Voldemort was going to kill Regulus Black, because of something called a Horcrux.
Peter Pettigrew knew something he knew could convince Voldemort to spare Regulus Black; Peter Pettigrew was the Potter's Secret Keeper.
Conclusion? Peter Pettigrew had to betray his best friends in the entire world. It was true that, in the painful mess of war, his best friends rarely spoke to him. Prongs and Lily were hidden away with Harry, but even before then, ever since they had gotten together, James hadn't had time for Peter. He had time for Lily and Padfoot, and a little for Moony, but they were better than Wormy. Wormy was nothing but the wet-eyed tag along. Moony, being the best fighter, the best dueler, was gone quite a bit, and Padfoot still didn't have a home, so Wormy couldn't really blame them for neglecting him, though it did sting. Peter had been left all but all alone, when one day, Regulus Arcturus Black had found him crying behind a wall during a battle between the Order and a handful of Death eaters. Wormy had been expecting to be killed then, but Reg had paused, recognizing his brother's friend, lowered his wand, and sat beside Wormy.
Wormy remembered the first time Reg had kissed him, the night after Harry was born. The first kiss had been soft and tentative, just a whisper of a hard, warm mouth on Wormy's. Then Reg had smirked, his face so similar to Padfoot's, his expressions so different. Wormy remembered exactly what Reg had said, and never would he forget. "S'this okay, Pettigrew?" Reg had always called him Pettigrew.
"More than okay," Wormy had responded, and then Reg's hot mouth was much less gentle, and Wormy had found that he liked that; he really, really liked that.
The first time that they'd shagged had been when Peter was crying after Prongs had picked him as the Secret Keeper. Regulus had lain Wormy down on his back and slowly stripped away what protected him, made him feel safe, including his clothing. He'd kissed the stretch marks on the chubby tummy, purple and fading from long term weight loss, and kissed him long on the mouth before taking him in the dark room of Wormy's Muggle studio apartment, slow and hot and loving, holding him as if he were a girl rather than how Wormy had thought it would be, like a dog.
And now, Wormy was standing by the Dark Lord in a dark room in dark Lucius' house, with Reg furiously looking at him in confusion. He saw Malfoy and his wife, both aware of his reason for being here. Lucius looked at Voldemort, but Narcissa looked at her baby Draco, as if wondering how an uncle could betray such a child. Snive- no, Snape- was sitting, glaring Wormy's way. As he aged, he felt less and less sorry for never questioning Prongs and Padfoot's attitude towards the boy; the older he got, the Darker he became, and so much less like the ambitious young boy from first year. Wormy considered the fact that it was their faults; the Marauders', self-righteous Gryffindors who were once so carefree that their biggest problems were the greasy haired Slytherin boy and where in Merlin's name Padfoot had thrown the Cloak this time.
"Good evening, my friends," Voldemort said, before pausing to look back at Pettigrew. "A Mr. Peter Pettigrew believes he has some information that will, perhaps, convince me to spare Regulus." Voldemort looked right at Reg, then. "So, little Wormtail, what do you have to tell?" Wormtail. Voldemort was mocking him, calling him by his Marauder name. He could only wonder if Voldemort knew what a slap in the face was. If he didn't, he would soon.
"I am the Potter's Secret Keeper."
"Pettigrew!" Reg exclaimed, pain bright in his eyes.
"Where are they?" Voldemort immediately growled greedily.
"Promise you won't kill Reg."
"Pettigrew, don't-"
"I promise I will not lay a hand, nor Curse, on your boyfriend, Wormtail," Voldemort spat, more malice on the name. It took Wormy all he had not to wince.
"James, Lily, and Harry Potter are-"
"Pettigrew-"
"At their home in Godric's Hollow." Wormy felt the spell break on the afternoon of Ocotber 31, 1981. Voldemort's face twisted into a horrifying, inhuman grin, and Wormy knew that he had lied. His stomach fell in terrified anticipation.
"Nagini," He spat once, greedily. Before Wormy could blink, Nagini the viper bit Reg all over. Wormy screamed, the biggest, most searing pain filling him as Regulus, his Reg, bled quickly out.
"Pettigrew… I told y…" Reg swallowed. "I love you," And then Wormy ran to him, hands in his slicked back hair, as Nagini slithered towards Voldemort. Wormy began to weep.
"I l-love y-you, t-t-t-to-too!" But Reg had only begun to smile before he was just simply dead, like so many others were, like James and Lily were about to be.
And then, Wormy died, too- the wet-eyed, affectionate, dorky boy from Hogwarts; the quiet, nervous teenager from Reg's dreams; the mediocre fighter from the war. Wormy was replaced by Wormtail, a loveless, prideless leach- and not for seventeen years did Wormy once again make an appearance, in the dungeons of Malfoy Manor, to save Harry Potter from the man he had once handed his entire life to, only to have it killed.