After the Underground was plunged into a finite winter, killing off 75% of Monster-kind. Sans, along with Flowey have teamed up to now take the soul of the eighth human and cross the barrier themselves to escape this frozen hell they've been living in for 8 years.
Sans no longer cares about Frisk, and certainly doesn't care about the life of who's now standing in front of him.
He won't let you pass the bridge behind him heading further into Waterfall, not knowing what lies beyond it.