"well pal, looks like it's the end of the line for ya." "cause i honestly doubt ya can escape this place before it explodes." "heh, once this fight ends, maybe i won't have to keep goin', eh?" "welp, lets get to the point."
this is the core encounter and the final encounter..? in this fight, sans is confident that even if he can't kill you, the explosion from the core will. so don't keep the inevitable waiting...