frankly i get that this ending can be perceived as obscure because it requires you to "banter" with Sans about it, but still.

i get that most people shift eyes to the Pacify and Ice Shock endings because talking to him sounds outlandish - but character behaviour is merely the bread, the motives are the meat, and the flavoring is the (re)actions.

you cant make a full burger (if you will, metaphorically) without those as a recipe for a character in writing. if they have no motive, you bought a bread loaf and flavoring, which barely has a taste, if any at all.

Ice Shock and Pacify don't give the motives, only laying extra groundwork on the behaviour and reactions.

he's not a random killer who kills for thrill, he is a person blinded by depression in a sense to the point where wiping the world as anyone'd know it seems like the most sensical idea to him, because he doesn't think about it at all anymore, and is deadset on that goal. thus you can look at "i killed suzie" in a perspective of "i'm going to provoke these people so they remain and fight me, and then i have all i need to erase this ridiculous, unfair world."

heck, this is a longshot because i dont remember much, but i'd wager that "to think you ruined my life and came back to ruin it again" is just self-victimization on his end to justify killing Kris, Susie and Noelle for power and wiping the world already, since - reminder, he never opens up about that. not even here.

EDIT: actually, he does mention it slightly while talking about the fear in Suzie's eyes. i am just forgetting something quite likely.