Undertale - so basically

There's a difference.
The genocide she stopped was on her own world. A few billion at most.
The genocide she caused was multiverser. ALL magic beings on ALL dimentions are now dead because of her. We can't put a number on this cause we don't know how many dimentions were made out of magic beings.

The genocide that she stopped was kickstarted by her mother and the citizens that wanted murder. And they got away without punishment.
The genocide that she caused killed who knows how many innocents in the multiverse.

Her answer, was a permanent short term solution to a complex long term problem.
And stopping a genocide by causing another one isn't a good answer.

Her actions in the finale pretty much resulted in an entire race (living beings made up of magic) being killed off.
Or at least it killed the Magic Council and a few other characters we knew, with fans and haters alike stretching that out to include several other characters that looked like they lived off of magic.