I had a similar thing happen with a male elephant killing a female, something that hasn't happened before or since. It was inside a very large indoor "elephant house" building in that zoo, and it was snowing out, so it's possible the male elephant got "trapped" in the corner and it triggered the overcrowding behaviors, even though there was plenty of room and he shouldn't have been trapped. I occasionally get a message that an animal is low welfare due to space problems in a habitat that isn't overcrowded and simply picking them up and moving them helps.
I have noticed that when the sex ratio is "incorrect," which often happens when a bunch of babies grow up and within seconds start fighting, often a female will attack and injure a male or vice versa, even if the issue is too many females or too many males who would realistically be fighting for position within their own sex's hierarchy. I don't know enough about most captive behaviors as opposed to normal wild behaviors to say whether this is actually a thing in zoos in a given species, or if it's simply random in the game. It's true that captive dynamics are often very different than what one sees in the wild when there is more opportunity for dispersion of sub adults or adults, and more space for animals to get away from one another.
In the wild, mandrils
live in groups that have multiple males, but the game only allows one adult male mandril per habitat. Same with chimps. And spotted hyenas, where females have dominance hierarchies, but troops have multiple adult females and an elaborate social network based on kin and alliances. I don't know if this reflects a difference between captive and wild social groups, though. When two animals within a social group have a conflict, typically one will back down or flee before serious injuries occur, but in captivity maybe they can't get far enough away from one another to end the conflict?
The male killing female thing could be a glitch, or maybe it's a low probability outcome of conflict with some species, but it happens very occasionally.