This. Alistair, in fact, is half-elf. Fiona is his mother. This was long theorized, but the way she coyly asks about him in DAI all-but-confirms it.
People who say "if a character evokes emotion, they're well done!" know literally nothing at all about writing. Not the tiniest shred. Any hack can write a pedophile serial killer and get them hated. I don't know how that terrible, awful piece of "wisdom" started floating around the Internet but it's one of the most ridiculous pieces of "wisdom" floating around it and that's saying a lot. Sera is the Inquisitor's ally, not a villain, she was not written with the intent that almost everyone dislikes her. Yet she's almost universally disliked outside of the "crazy feminist who thinks Elizabeth Bathory was just a strong independent woman and you're a misogynist if you hate her" demographic. Sera is a poor character and the fact that the majority of the playerbase seems to get less than no enjoyment out of her as a companion is not a positive thing.
Realism will always, and should always, take a backseat to what makes the game more enjoyable. Yes, there are people like this in real life. That doesn't mean there should be a character from every demographic. There's also pacifists out there who won't help me fight the enemy, I don't want them in my group. There's hardcore vegans who will leave the group if I do that "hunt some rams for meat for starving refugees" quest. Don't want them either.
True hatred takes the form of pure ignorance IMO. A serial killer character etc. you mentioned doesn't get any emotion from me and I simply ignore it because I know fundamentally there is nothing to see there. A failure to acknowledge at all is probably the strongest indication that there is nothing interesting or cool about a character to a person.
Generally if someone takes time and energy to actively comment on something, like this, it's because they find something going on they don't totally understand... which suggests that actually yes the character creator did a good job. I know it's the same person that created Aveline and my god she was boring and got nothing out of me, despite the claim that some people liked and some people disliked her. This kind of 'active hatred' practiced by actively engaging the thing while declaring it's just nothing and a meaningless horrible character is pretty hypocritical.
You could say the only way to prove you truly don't find anything challenging about the character is not to comment at all. The moment you enter the conversation suggests that there is something you don't grasp about the character. No one is commenting on Bathory because there is nothing not to grasp, it's just a serial killer, as you said.
I personally was only interested in DA:I for Sera basically, and so am delighted to see so many people struggling with her character. I don't know the details and whether or not she really ultimately is just a boring hum-drum serial killer, but I definitely think that she provokes such strong opinions is interesting.