I know it's been analyzed to death at this point, but I rewatched the Sans fight and it's only now occuring to me just how... weird it is to witness??
Like we can talk about lore implications all we want and it'd be fun but when you take a step back for a moment, it's really just. Sans venting at you for about ten minutes straight.
Everything he knows about the timelines, his nihilism regarding resets, going out of his way to call the player out on their motivations multiple times. It's like everything he was repressing in all the other timelines finally broke to the surface, and he still somehow manages to be fairly composed with his word choice.
(He's significantly less composed when he's throwing you around the battle box as fast as he physically can until he just can't do it anymore. All the more evidence that this fight is just him losing it)
But here's the thing. He's also doing his best to get you to give up. And as much as I applaud him for breaking the game's mechanics wide open, disabling invincibility frames and attacking you in the menu... he really fails at making you want to give up.
Because as much as he claims to know "our type," he forgets one crucial detail: we love being told things.
He understands that we're doing the No Mercy route just because we can, but he neglects "curiosity" as a motivating factor. We want to learn more by exploring different routes, and in his frustration, Sans gives us exactly what we want.
(We also get a challenge from him, and what gamer dedicated enough to get past Undyne wouldn't enjoy that)
And that's kind of. Sad, in a way?? Even when he locks us into his turn, he can't stop himself from talking. Everything he does to make us frustrated or bored doesn't work because despite everything, we still like Sans. We want to hear what he has to say. And he has a lot to say that he would normally keep close to his chest otherwise.
I don't know if that's what the dynamic is meant to be like in canon, given how ambiguous Frisk's motivations are and how we don't technically know if there's a "player" involved in the first place, but that's just how it reads to me. And I think Sans having this slightly flawed perception of what the anomaly wants from him is a really interesting angle for his character and the fight as a whole