It's not that people want Hydaelyn to be a "bad thing," it's just that it seems rather apparent that she's possessing Minfilia. Even if Minfilia went with it willingly, the Dude still does NOT abide! That is not the behavior of a benevolent, all-loving mother goddess! (See, this is why I hate dealing with deities...)
Seriously, just let me beatthe shit outtasome sense into her. That usually works in these kinds of stories.
Some people might want Hydaelyn to be "evil," but they're weird.