That whole scene is in the genocide run word-for-word. You become so terrifying that Flowey calls you a freak.

The spoilery comic yes.

You can do the entire run safely up until the point AFTER killing sans. You can safely kill sans, and then reset after that. If you continue to Asgore, then your game data gets a little bit ****** . There are ways to ******* it, though.

by the time you get to Sans, if you're still on the Genocide route, you're past the point of no return. I think the Mettaton fight is the last chance to abort

That's not what he meant, at the end of the Genocide run (LEGIT SPOILERS BTW): Chara approaches you and destroys the world. This forces the game to open up to a black screen, and the game won't work again unless you sell your soul to Chara. This means that you can never truly get the True Pacifist ending, because Chara will take control of Frisk's body after the credits. There is no way to reverse this unless you know which files to delete inside of the actual game data.