Lusamine was scary.
I mean, there's been a few nutcases among the villains, but most of them are of the "reshape and/or conquer the world" brand. I can deal with that, we all have moments like that after all.

Lusamine though?
Yeah no. Thanks.
Too realistic maybe, ignoring the part with the transdimensional alien poison.

Though even before the reveal she disturbed me more than the other villains.
Sure, Team Aqua/Magma's plans were apocalyptically bad ideas, Cyrus seriously needed a hug and Lysander was mayhaps a tad extreme in his endgame, but neither of them had those Joker-grade crazy looks.

By the way, worst part of USUM? Not that they altered the story, but that they basically chopped the original story in pieces and then frankenstein'ed the new parts together like the fossil lady in Galar.
They kept all of Lusamine's crazy- the Smilex-grins, her rants about her kids and the MC, her friggin collection...
And then? Nope, she was the good guy all along.
It just doesn't work.
Like the Galar-fossils.