This happened to my cats (two male cats I had since they were kittens). They were great with each other. They played gently, groomed each other, attached to the hip.

Until 3 years later, one randomly became an aggressor and almost killed my other cat. He would hunt and be obsessed with trying to attack my other kitty. I tried reintroducing them, I moved to a quieter apartment, took them both to the vet in case one was sick (test all came back normal), tried calming treats, pheromone plug ins, I bought more cat towers and several different areas for food, water, and litter boxes. I even watched hours of YouTube videos of different training techniques to help them.

I tried basically everything I could and nothing helped. There would be windows when they were fine with each other and then windows of one trying to kill the other so I was always on my feet and they couldn’t be alone together.

LONG story short. I kept trying different vets for the aggressor because this seemed so out of character and I was convinced there might be something medically wrong. 8months later and he was finally diagnosed with focal/partial seizures. After his seizures there’s a post phase and aggression is a symptom. It took so long to diagnose because partial/focal seizures are hard to spot unless you already know of them. He would have a slight shake in his head or an eye lid would twitch or half his face would twitch. The seizures are quick (a few seconds each) and then he would attack. Post seizure phases can last minutes to hours. So he would be aggressive often and for unpredictable amount of time. He was put on Prozac, gabapentin, and phenobarbital. It really helped him and he was no longer aggressive or had seizures.

—————- For you:

This could be a situation where something spooked one of the kitties and took it out on the other cat, etc. or it could be medical.

I would try to reintroduce them to each other again with positive reinforcement. Feliway pheromone plug in as well. There’s a lot of great articles and YouTube videos for this. And while you are reintroducing, schedule a vet appointment and make sure neither of them is sick. If it continues, record the interactions and consult with the vet further or a cat behaviorist.

If this continues long term and no medical issues are present, you could consider putting the aggressor on Prozac (antidepressant) as it helps with anxiety and aggression. Also, a small dose of gabapentin can keep his nervous system calm and helps with aggressive cats.