There is a theory, another storyline. I made my view of the story change completely.
Sense bill is able to return in a new form, he could've taken the form of stan, to defeat bill in the end. When the axolotl is called by bill, he can live again, in a different time, and a different form, so it's possible for him to come back to the past. also, the axolotl states "misses home and can't return. says he's happy, he's a liar". and to me, it sounds like stanley. so he could've taken the form of stanley pines. Bill takes the mind of stan, and saves the world from a past version of himself. This basically creates a time loop, and it goes on forever. There are many parallels between stanley and bill. Bill goes back to the past to redeem himself, by defeating himself.