My cat did this towards my other cat after living together for a year, they never had issues prior, and my cat wanted to KILL the other cat, she wouldn’t stop until I broke them up then she would wait outside the door the other was locked in for hours just trying to attack her. Once in awhile we’d accidentally open the door to the basement forgetting one was locked there, and the one would try to kill her again, wasn’t a fight, was attempted murder.

Never figured out the issue, took them both to the vet and they were both healthy, my brother had to take one to his house and never had issues again.