Fimir are not bound, utterly, to humans for such things, no. However, at the same time, they are, in fact, an ancient, debased, evil race. I did mean what I said in this post:
Chaos itself, the main faction at least, in the form of the Norscans, the Chaos Warriors, the Daemons, these can be varied, even self-parodying things on occasion. Edgelord level stuff that you can't help but laugh at on occasion. But...they are evil. It can be fun to pretend to be the bad guy, on occasion. To play the Monsters, to use the terminology I've seen others use when regarding certain Warhammer Quests. Heck, Eliphas in Dawn of War is great, barrels of fun listening to Warboss Gorgutz gab. But while on occasion interpretations of Warhammer can soften it, to twist a bit away from the grimdark to more noblebright, to do things without going to grimderp, sometimes the source material does remind that there are truly, awful, awful things that live there. Heinous acts. Rituals of such depravity and darkness that even many others in the evil faction sometimes are shy of performing. The evil of technology, in the form of the skaven, introducing things like warp nukes and what have you, pollution in the earth. But when a band of Slaaneshi worshippers desecrate a place and its peoples, it is the point of their entire existence that there aren't limits to what they'll do. It's up to the writer to chose how to write that, hopefully not necessarily ever having to show it fully directly, that seems like some twisted kind of one-handed writing. Greenskins will kick a child's skull around as a toy, beastmen will eat a baby because they find it delicious and tender, Druchii will cut off a man's face and show it to his lover before clapping her in chains - horrifying job, Malus Darkblade - and so on and so on.
At the end of the day, no, they don't absolutely require it. They are not reliant. The Meargh is the ruling head of each clan, and she pops out the eggs as a reptile does, and any lady eggs spawn and then leave to form their own new clan. But that it doesn't mean it absolutely doesn't happen. The Fimir are a decaying kind of monsters, a receding shadow, and faded out from the view of the Chaos Gods and GW as time went on as they were shoved ever more into the background.
And, I will admit that yes it's awful, and reprehensible, and I've daubed more than a bit of grey or outright light into a lot of things before, but sometimes, in fantasy - especially Warhammer Fantasy, there are monsters that are bad that need to go down that do bad, awful, monstrous things and will just...never stop. Because they like how they are and what they do. There are monsters in human skin IRL that are like that. Some, we know about. Others, we don't. Sometimes, even though we do know about them, they aren't stopped.
Here, they can be. I could, I suppose, write a massive plague that would wipe them all out or something. Heck, anyone can do that, anytime they want, about anyone, or anything, they don't like. About the greenskins, who stop being funny hooligans when they reach critical mass and violence. About the skaven, who are goofy and treacherous, except when they're unified by the Horned Rat to do things like blow up the moon and wipe out entire nations as per the End Times. And so on. But that's not...really...going to be a thing, right here.
Here be true, gleeful, hateful, unrepentant monsters.
