His mortality terrifies him because it shouts in his face that he is ultimately no better or different than the poor. The great equalizer comes for us all, and most of us will be forgotten within just a handful of generations as though we never existed at all. That's easier to reconcile with when you already know that you're not special. Peter spent quite a long time believing otherwise. With every passing year, his desperation grows. A decent guy would use his absurd resources to improve life for all, cementing a positive legacy, feeling more fulfilled; he's a quantity over quality kind of guy though. Fewer, happier years are not as attractive to him as maximum, if miserable, lifetime.