I've run some very hateful NPCs in my 28 years of gaming, and most of them have been a product of the campaign - evil wizards, nasty warlords, insane pirates. Character archetypes you would expect to be evil.
I'm now running a Call of Cthulhu campaign in 1933 and I've created a couple of characters I wanted to be distinct, especially a janitor at Miskatonic University called Lewis. He's not evil (although his opinions are), or insane, and has no plans to take over the world. He just mops floors and cleans up after the students. He's also one of the most hateful people you'd want to meet - he's sexist, racist, homophobic, anti-semite, willfuly ignorant, offensive and pretty much everything you'd dislike in a person. I created him as a red herring for the PCs, a man who seemed to hate everything and everyone and so might have been responsible for the apparent hate crime incidents happening around the campus. They only spoke to him a couple of times and then managd to follow the clues to the real culprits. During those conversations he would spit and moan about anything the PCs questioned him about. He blamed it on the kids, the women, the foreigners, the coloured folk, the Jews. His language was colourful and his attitude horrendous, and if he could whine and gripe about it he would.
This was the first time I'd portrayed a character like that - I'd used hateful characters before but never to this degree - and he was successfuly nasty. I think the PCs were trying to find a reason to beat the living shit out of him, but as he was in his sixtes and one of the players was a cop they bit their tongues and just chewed him out. More than that, though, throughout the entire encounter, the looks on the player's faces changed from interest to disgust, and I wasn't sure if the expressions were aimed at the NPC or at me for running him. I felt horribly uncomfortable playing the role and I've decided not to run an NPC like that again. In fact, I dwelt on the character and how I must have sounded at the table for a few days after the session and wondered if I should have apologised at the table, or mentioned that I didn't share any of this bastard's views.
It seems that it's okay for me to play NPCs that want to enslave the world with magic powers, invade star systems with fleets of starships and decimate millions, or have the PCs kinapped and beaten by psychopathic pirates - all things I'd never do or contemplate. That's all part of the story. But the moment I play a character that may have actually existed, and may have shared the views of people living in the real world today? It made my skin crawl to inhabit the personality of such a person and I don't think I'll be doing that again.
Playing in the real world - or, at least, a version of it - is far, far scarier than anything I've done with the fantasy and sci-fi settings I've played in. I think that's what gives Call of Cthulhu it's power.