Undertale - absolute power corrupts absolutely

Sans tells you that if you guys are friends, then you won't come back, but maybe that doesn't mean "to the game, at all".

Maybe Sans kills you after you spare him in the hopes that you'll do a reset, abandoning your Hitler route.

Honestly, once you've beaten the pacifist route, watching other people play is a lot more fun. A good LP has all the benefits of seeing the characters again plus the added joy of seeing someone else experience the game for the first time.