To clarify, Sirius was NOT crushing on his cousin in the last chapter. Eww.
Peter Pettigrew was a coward.
He knew it, everybody else did. So why deny it?
It all started off in his 7th year when Marina Parkinson (now Edgecombe) took him to that broom closet and, er, convinced him to spy on his friends.
He tried to back out, but he knew death would be his reward. So instead, he hung out with James, Remus and Sirius by day, and Lucius, Bellatrix, and Rabastan by night.
He thought he had good reason to join the Death Eaters. After all, the rest of his friends had no use for him. Sirius was off bedding girls left and right, Remus was drowning in self-pity, and James was too busy being overexcited about Lily becoming his girlfriend. Each attempt to talk to him was either making fun of him, or asking him to do something.
So yeah, blame him for wanting power, recognition, and friends.
He didn't know what he was getting into at first. But when he killed that first defenseless Muggle, he knew he was in deep. He thought it was atrocious, but death after death, rape after rape, torture after torture, he was happy.
Happy that he had some power in the world, that nobody called him a pushover when he had those black robes on, and the feeling of usefulness that came when the Dark Lord gave him the Dark Mark.
Of course, the little voice in the back of his head was saying that this was all idiotic and he should just run away or go to the old goat. But as usual, he ignored it.
Then, when the Potters made him their Secret-Keeper, he wanted to scream at them for making such a horrible decision.
When he heard of their death, he ran.
Flash forward almost 18 years later and he was far uglier than he should be. Even though that was the least of his worries, he knew he wouldn't have much time left on this world, seeing as Potter was bound to luckily get a wand.
And when he went into the basement, he felt pity and regret. Pity on the Potter boy, and regret for what his life had become.
He didn't think that his own arm would choke him to death.
One of the perks of being a Death Eater.