You're just wrong here. You said there's nothing to suggest she's a genocidal maniac, yet she committed a very blatant act of genocide (wiping out an entire religious group) that resulted in the deaths of THOUSANDS of innocent people.
She didn't just kill the people in the sept, she killed everyone in the whole area with absolutely no regard for innocent lives. We see the bell crushing innocent people, and the explosion rippled out pretty far from just the Sept itself. It's not like she just poisoned the faith militant like Arya did with the Freys. She blew up a holy temple in the middle of a crowded city with absolutely no regard for the lives of her own people.
That's the difference in what she did and what the others did/have done. This wasn't attack against another people, these were ostensibly her people, in her city. And it wasn't an assassination, or a targeted strike, or it wasn't like she was trying to only get a select few people and something went wrong. This was the plan.
Now, have there been other rulers that have done horrible stuff, sure, in the past.
So for #1, it doesn't matter what other people have done. Your premise is whether her winning would be any worse for the people than any of the others winning, and she is CLEARLY worse than everyone else in contention here.
She's a genocidal maniac even by the standards of this time.