Not everyone's past has been a happy one.
Just like people, a bad experience in the past can drive your present/future behavior.
I've experienced this first hand with the first cat we had. For her first 9 years, Pruts lived with a slightly older male cat who basically owned her (up to a point of physical abuse by the male). To make things worse, the male was the owner's favorite cat, and she wasn't subtle about not liking Pruts (who we took in when the owner wanted to get rid of her).
After a few months, we took in a second cat, Luna. 6 weeks old, still looking for a motherly presence. And Pruts beats her up.
Some cats may not get along, but this was different. Pruts would get violent whenever Luna revealed her presence. She wasn't allowed to eat, drink, use the litter box or interact with us, or she'd face a beating (unless we intervened of course).
When we realized that this was never going to stop, we tried to see if we could find a new home for Pruts, keep Luna and add a different cat to our home. We brought in a 6 week old kitten (from a friend) to test it. Within minutes, Luna beats up the new kitten. Luna didn't even wait to see if the kitten was friendly.
She repeated the exact same behavior. Luna kept her distance from the kitten, but would actively attack her if the kitten ate, drank, used the litter box, received attention, ...
Luna's behavior proved to me that violent behavior can be learned from past experiences.
The past cannot be changed.
This means that we might never be able to find out why your female is beating the younger male, since cats are unable to tell us why they're doing what they're doing.
Some possible reasons might include:
- Being abused by another cat in the past (cfr Pruts, Luna)
- Being jealous of anyone else getting attention from the humans (cfr Pruts)
- The younger male might even resemble the cat who abused your female.
- Some cats just don't like other cats and prefer to be alone (stands to reason, since cats are territorial)
But what about the present/future?
Since some of these reasons (e.g. experience with being abused) can never be undone, you should also consider that it's (possibly) irrelevant why she is now violent towards the younger male, and we should instead focus on the fact that she is violent towards him.
In other words, regardless of whether she has a good reason or not, she should stop attacking the younger male.
When the test with Luna and the new kitten failed; we decided to keep Pruts and Luna (they were both "damaged" in a way that we felt bad abandoning them). Instead, we put time and effort into making their life situation bearable.
This basically turns the question into "how do you train a cat?", which is a very broad topic. In short, the rule of thumb is "if you misbehave, you get punished (= inconvenienced)".
If you keep this up consistently, then the cat will start to realize that the two (being naughty and being inconvenienced) are cause/effect, and they will then avoid doing the thing that always leads to the unwanted consequence.
Some examples of consequences:
- As a first warning, say the cat's name to get her attention.
- As a second warning, say "No.". Repeat this until you feel that the cat is ignoring/not understanding you. (But even if you know she doesn't understand, it's important that you still say it. Eventually, she'll see that "No" is a precursor to further escalation from you)
- As a third response: a spray bottle, set to release a jet (not a mist). Aim it at the cat. You don't even need to hit it, just make them feel inconvenienced enough that they walk away from it.
- If they still misbehave, directly step towards them and physically (but calmly) remove them from the room. (If they walk away when you approach them, do not chase them. They obeyed you by moving away, that should be rewarded by you not chasing her).
Over time, you'll see that the cat picks up on the consistent pattern, and will yield to you before you need to even pick up the spray bottle.
Don't treat her like she's the devil incarnate.
However, one more important thing to consider is that your female cat's response might be justified (in her head). If you always treat her like she's the problem, then she might grow distant from you (as you always oppose her when she does what she thinks is right).
I highly suggest you compromise with her from time to time, to prevent that from happening.
For example, Pruts was very insistent on eating first (likely because her abuser did the same to her). Since Luna was not a picky eater, we decided to compromise and let Pruts eat first.
It's not fair to Luna on principle, but in reality it removed what would otherwise be a daily cause for starting a fight.
It took over a year, but we managed to finally get a truce. Pruts and Luna lived separate lives in the same house. They shared nothing but a litter box.
On one hand, it's sad that this is the best we could get. Everyone wants their cats to get along, especially if you consider them as family members.
On the other hand, a truce is at least livable, whereas an abusive relationship is not livable for the victim.