He forgets sometimes that Napstablook and Mettaton even know each other. He's not sure how that even happened. Napstablook never seemed to leave their snail farm all that often, and Sans can't see how Waterfall would appeal to someone like Mettaton. All that damp must be hell on circuitry.

But they do know each other, and they clearly care very deeply about each other.

There's that thing about choice again. Maybe that's the best way to think about it. Someone can choose to do something terrible, but you don't have to choose to let them. You can choose to try and stop them. Or stand in their way. There's some subtle difference between making your own choice and taking someone else's choice away; a line that's easily crossed.

"that makes...sense, i think. about choice."

He needs to think about it more.

He rubs the back of his skull as Mettaton continues. He's not sure Mettaton should be telling him this. History has shown that Sans isn't so great at keeping secrets that aren't his own.

A suicide pact. Like Frisk and Chara. No wonder Mettaton is drawing a comparison between what happened with Napstablook and what happened with Sans. Telling someone you'll die with them--that's pretty manipulative. But it seems like it worked.

"i saw them on the beach that day. they said they wanted to..."

To just swim out until they reached the edge of Wonderland, and whatever happens to people who try that, happened. Sans had tried to talk them out of it, in his usual apathetic, half-hearted way. Guess it hadn't worked. Should've tried harder.

"...it's good you got them to stop."