People sometimes forget that cats are still wild animals at heart. That's why it can be a surprise when a cat suddenly acts aggressively. Rest assured, this kind of behavior doesn't just materialize out of thin air. There is always a cause. Matthew G. writes:
Matthew, you were right to have Sabi checked out by the vet. Sometimes behavior changes like this are brought on by chronic pain. We still wouldn't rule it completely out, but if he got a clean bill of health from the vet, it's more likely that something else is bothering him.
We would be concerned about his senses. If he's having difficulty seeing or hearing, he may act out in frustration. The pacing, moaning and hissing when alone could be signs of sensory problems. The behavior you describe sounds like a much older cat who is losing his senses as well as his mental faculties. Do you know for certain that he is only 6-7 years old?
We would also not rule out scents that are beyond our human perception. If all of the cats are allowed outdoors, it's possible that his sister has attracted a suitor. Yes, it can happen even though she's been fixed. If she returned home with a strange cat's smell on her, that would be most upsetting to Sabi. To make matters worse, she could be leaving the stranger's scent around the home. Feral cats are even more territorial than those born among humans and a strange cat's smell could drive Sabi absolutely bonkers. We've even known a cat who rejected a litter mate who returned from a veterinarian's office with another cat's smell on them. If that's the case, the best course of action would be to isolate the two cats from one another until the interloper's smell has dissipated. A bath could help but it could also create new stresses from the new smells it would introduce. Even if you were to settle this problem now, if you continue to allow the cats outside, it's likely to happen again.
Unfortunately, all we can do is venture guesses, but hopefully they'll give you some ideas you've not considered before. Cases like Sabi's often require vigilant detective work and an eye for the cat's point of view on their world.
We wish you and Sabi all the best!