Board Thread:Questions and Answers/@comment-33450064-20171022193730/@comment-25398349-20171229152257

Screw it, I’ll necro this. Asgore (who, btw, isn’t a mini-boss) is like the other boss monsters you fight, due to your violent murder spree, you’re much stronger than you need to be, therefore you kill him.

As for the second part, Sans isn’t even the only Genocide boss, you had to beat Undyne the Undying to get this far. As for why this time Sans does attack you (despite the promise to Toriel), you’ve murdered everyone in the Underground, and if you kill Asgore and escape with his SOUL (or worse, the 6 human SOULs) then you’ll proceed to kill everything else.