Why is this one female mosquito fish being so mean to my other fish?

First of all, you have way too many fish in that tank! Feeding them to a predator is less cruel that cramming them all in that tiny tank. Besides, big fish eating little fish is no more cruel than when you eat a hamburger. uming you are not a vegetarian, do you feel a little bit of hate for that cow when you take a bite? We don't eat red meat to be cruel. Hopefully you see that it is our nature to eat cows, just like it is the nature of big fish to eat small fish. To answer your question, sometimes female livebearers can appear to be aggressive, but you said no damage was being done. She is most likely displaying breeding behavior instead of aggression.