'Don't say I didn't warn you...'

[vømy]

After you cut Sans in half, he says some final words. However, one phrase caught my eyes, 'Don't say I didn't warn you...'. And that puzzled me. What would Sans be warning you about? By the way, I'm not good with stuff like this.

[Dani]

After you kill Sans, you literally erase UNDERTALE itself, and sell your soul to Chara.

[vømy]

Thanks. I kinda answered myself with the Chara scene but I wasn't sure.

[evil silver]

:O Noice. Also, if it wasn't a canon thing in UNDERTALE and it was just something there to puzzle players, I'd say it's a warning on if the 'Alternate Universes came into play' and that Sans, even if going Genocide, would be warning you about Underswap, Underfell, Inktale, Errortale etc. happening. Speaking of Sans, my friend apparently knows a cheat/hack to make Sans look like ANY Sans, so I told him to make Ink... xD