All Peter can do is watch, the Cat purring and the weight of him against his leg offering some measure of comfort. He's aware that he's crying, that his face is wet, but he feels so helpless, he can't do a thing and it's his fault.

--and when Stephen finally stops, there's a sense of finality that he just doesn't like.

"No--no, you gotta keep going! You-you can't stop!" He knows that it's too late, but what if it's not? "You can't stop--"

Because stopping meant she was dead, it was over, and it's his fault.