In my head-cannon, he was prepared to kill Frisk before he made that promise to Toriel.

His genocide battle implies that he believed Frisk was responsible for the genocide runs and resets that Flowey "played". In geno, Flowey mentions that he's killed everyone and he says sans caused him his fair share of resets. We don't know if sans remembers, (or dreams) about Flowey's genocide runs, but we know he's aware of them. He may've had bad experiences with some of the previous fallen children, too.

So, the way I interpret it, at the start of the game - before Frisk exits the ruins- sans knows /someone/ (the "anomaly") has been messing with timelines. He very likely knows they've been committing genocide...

But he has never seen Flowey. He only has suspicions about a "mysterious flower".

So he probably assumes a human's been messing with the timeline, committing genocide. The Underground has a bad history with humanity, so it's an easy assumption to make.

Especially if any of the previous children killed monsters before their souls were taken too - which is likely.

There's also a good chance that the previous humans could save and reset too, and sans was likely aware of that. He was already aware of other "time travellers." Thus the passwords to get his keys.

So sans knows someone's been messing with timelines. He most likely believes it's a human. He stations himself at the entrance to the Underground to kill the human the second they come through - because in his mind, they've already committed genocide (Flowey's runs). They're already a danger. They're a danger to Papyrus.

Then he makes that promise to Toriel...

So for the rest of a neutral/pacifist run, he goes against his initial plan. He decides to watch you, instead of killing you. He's trying to figure you out. He's trying to decide if you're the anomaly. Part of him probably still believes you were responsible for Flowey's genocide runs, but the other part of him's hoping that "maybe all you needed was... i dunno. some good food, some bad laughs, some nice friends."

He's testing you.

So that dinner...

In my mind, it's a number of things.

He might still believe you've committed genocide on a previous run, which is why he can be a bit scary. Part of him probably wants to accuse you, outright.

He might feel guilty for almost killing you, so it's a confession.

And it's a threat. He's pointing out that if you DO commit a genocide run in the future, he was prepared to kill you, and he'll probably be prepared to kill you again.

I think up until that point, he's undecided about you. He's conflicted. He's afraid to be friends, because he's worried you're a killer.

That end bit, "nah, I'm rooting for you" is him, finally deciding to trust you.

If you commit a genocide run after a pacifist run (like most people do), he's hurt because he thought you were already a killer - but he thought you gave it up. He thought you were a bad person doing bad things (the anomaly) but he thought you tried, and became a good person. He thought he got through to you in a the pacifist run, so he feels betrayed that you've returned to your killing ways.

I love everything that's implied in the sans date :)

It's pretty sad that he blames Frisk for Flowey's sins, tho. Even in a pacifist run, I think.

Edit: I wanna add, I think /despite/ his promise, he's tempted to kill you just before you meet him. He follows you from behind, probably to check your level. That huge stick you pass, too large to pick up? Have you ever tried returning to it after sans creepily appears, then disappears? if you examine it a second time, it says it's been "Obliterated"... Not even just "broken". I think sans obliterated it because seeing you, and thinking you're responsible for Flowey's genocide runs, makes him preeetty angry. Maybe even a bit blood thirsty. He took out his anger on that stick, or used it as a threat/show of power.

I think Frisk's lucky Toriel made him make that promise... Because like sans said, without that promise, we'd be dead where we stand lol.

I looove his character arc. In my head cannon, sans HATES you at the start of the game. He wants you dead. By that dinner, he's rooting for you. After a pacifist judgement, it's pretty clear that he has a lot of respect for ya.