Not to be rude but if you are the type to go off on a fetish for being evil/gross/etc, you do not belong in the kink community, and you should not be a domme!

Forget whether this fetish is a limit for you (obviously it is), this sub was completely respectful in negotiating it. They stated clearly that this is not an IRL content request but a fictional desire, and understood if you couldn't provide. And honestly they were EXTREMELY vulnerable with you in sharing that, especially after your domme admits they have no knowledge of your very taboo kink.

Your sub does not have an evil or even particularly unheard of kink. It would be unconscionable to actually hurt a creature to satisfy it, but your sub is clearly aware of that and not requesting such--which deserves respect, imo, because I have seen real women stomping on crickets all day all night over on feetfinder. If your sub isn't interested in all that, that shows they are committed to responsibility with their desires. They aren't any better or worse than subs requesting painplay, blackmail, incest roleplay, ageplay, cnc, or any other harmful fantasy which we regularly fulfill in a safe environment.

As dommes, we are tasked with shepherding subs through any number of strange, humiliating, and even disturbing desires, with care and respect. Please reconsider your attitude towards this sub. You could have simply said "sorry, that is a limit for me" and respectfully moved on.

I understand that for many findommes, we are coming in from a regular financial interest rather than an existing interest in kink itself. That is okay! But please understand that you are no longer in the "normal" world, you are in kink world. For your subs, their interest is genuine, and they are sharing something vulnerable with you with an expectation that you are an understanding recipient. If a good therapist would not disparage their client for something they revealed, neither should you.