(fake cheerful, undertones of a psychotic killer veiled beneath a cheap smile) "Why, you’re in the Underground, of course! (pause) That’s unrelated. Anyway-"

(he's starting to lose his cool and peek from behind his disguise) "No! I already told you, you’re in the Underground! (pause) ...you seem to have missed the point. You were supposed to catch them idio-friend! Let’s try again!"

(your plan is finally working and everything is going your way) "Heheh, simple! I stole the human souls while you two were fighting! And now.. I’ve almost become a god! Once I get your soul I’ll destroy this world and then move on to the next! But that won’t matter to you… because you’ll be dead. Now, time for me to get your soul and FINALLY gain my TRUE POWER!"

(everything is starting to fall apart around you) "Stop! This isn’t right! I’m supposed to win! (pause) NO! YOU CAN’T DO THIS! THIS ISN’T HOW THE TIMELINE SHOULD GO! (pause) STOP!"

(you've just been defeated and you can't understand why Frisk is doing what they're doing) "But.. But I tried to kill you…! I ruined your “happy” ending! (pause) That’s.. So stupid! You know I’ll come back, right? I’ll kill you! I’LL KILL EVERYONE YOU LOVE!"