So, I've been taking the Neutral Route. And after I said that I'm not responsible for the power, after he says "ok, i'm not judging you for that. d i r t y. b r o t h e r. k i l l e r" the battle begins, and the battle looks very similar to Undertale Last Breath Phase 1, but before he starts fighting, he says with empty eyes "burn in hell." then he starts fighting, and all dialogs are SLIGHTLY changed, after the fight, he says "hey kid, mind taking a break? i would certainly use one right now. look, i know this is gonna sound stupid. but, maybe you just a bit of acceptance. come on, buddy, chum, friend, kiddo. i remember you, and you remember me, right?" then I'm required to spare him or else the game switches from Neutral to Genocide. He doesn't dunk on me, he legit spares me too. If I kill him; then I can only get Soulless Pacifist, and I want to get True Pacifist. I'm required to kill Papyrus to be able to battle him. In the call, he doesn't forgive me even if I spared him. All he says in the call is: "burn in hell idiot." I don't know how this happened in the Neutral Route. Edit: Forgot to mention that "Not a Slacker Anymore" plays for the battle music
You are not playing a true version of Undertale. In Undertale, basically everything you have said happens in this question doesn’t exist in the true game, such as a neutral route fight with Sans. If you pirated this game, it is possible that they gave you a faux copy. I would recommend removing the fake game and downloading it from an official source such as Steam.