Threat2 - Tumblr Posts
Opinion - Sans wasn't threatening you on that date

The iconic line. It's shocking, it's unexpected, it comes from the last person you'd expect it to, and it's fucking terrifying.
But I don't think Sans really intended it to be that way.
Look at where he's facing. He turns AWAY from Frisk when he says that. Sans wasn't trying to threaten you, as one person said he's just darkly stating what could have happened had he not made that promise to Toriel.
Afterwards Sans says to lighten up, which is likely in response to a negative reaction from Frisk. Wouldn't really make sense to do that if he wanted you to feel threatened. Then he tells you to 'take care of yourself, because someone really cares about you.' capping off the conversation with a reassuring message.
Unfortunately, he couldn't have accounted for the existence of sudden music cuts and talk sprites, which kinda ruined everything.