Yeah, it's a very clever move that plays into his recurring theme on our own brain-wired tendencies to distance ourselves from ye olde monsters on the screen(Or at least that's how I interpreted it?), in some cases to simply kill because the alternatives are too inconvenient. It really threw the rug under my feet the first time. I personally killed Toriel, Undyne, and Mettaton for that very reason.

He said you could get a better ending on a pacifist ending? huh. o_O You didn't start with a renegade run first, so I'm not sure what would cause that...very odd. Anyway, congrats on beating the true ending

When the game asks you to name the fallen human, the human you're naming is the one that died years ago. Remember, fallen down is used as another word for died in undertale, that's what alphys means in her notes when she says she gets monsters who have "fallen down". Additionally, the human who you name was the adopted son of Toriel and Asgore, and Asriel's closest friend and brother. However, some people have taken evidence found via the tapes, and through alternate evil playthroughs, that the character you name was in fact evil. That they wanted power and wanted to kill everyone. I myself disagree with this notion and think the human was simply colder then most others, and their plans were to get 7 human souls to free all the monsters. After all, he and asriel in the tapes talk about onky needing seven, and there's no way the fallen human would poison themselves to get their soul to asriel if all they wanted was power. If anything, they'd kill asriel to claim their soul. Regardless, the missing components of lore are hidden within the genocide route, including how frisk is the reincarnation of the fallen human, whose canon name is Chara. For me, the actions of the genocide run player in farming all monsters to death is what makes the char so evil at the end of the playthroughs. Oh well, regardless, there's your elaborating answer. Hope ot helps.