I've just gotten around to playing a game with IFs in it. I'm glad mine aren't the only ones spawning with horrible traits. One sim of mine has one that still hangs around her as a YA, and its traits are evil, mean-spirited, clumsy, angler, and dislikes children. She has all neutral and no mean traits, and this thing has been awful to her! It's actually slapped her around and then before she could respond, it would go into doll form where she'd have to apologize to interact with it again. (I tried to have her slap it back, but it walked away and put itself in doll form before she could.) As a teen it booby-trapped everything in the house that it could. I'm pretty sure that's when it got the evil trait. She got along with it fine as a child, except for the occasional scaring or mild argument, and the only trait she knew it had then was mean-spirited. It developed the dislikes children trait as an adult, as she learned the other traits when it was a teen and I gave her the observant reward to find out what they were.
One time when she was arguing with her IF, the maid was making her bed and made the "cuckoo" gesture at her while thinking of her in a thought bubble. It was hilarious!
I should probably have her tell it to leave her alone, or just let it stay in doll form without having her apologize to bring it back, but I admit I'm amused by the drama. She's a few days from adult and still has Pat (the name the game gave it) hanging around. The only way they stay on speaking terms is if I cancel out the negative interactions it tries to start and have her play foosball or video games with it.
Her older brother has an evil IF, too, but that's its only really negative trait, and he's charismatic, so they get along pretty well despite the frequent "scare" socials the IF pulls on him.