I'm sorry to hear that things aren't working out. I can relate to having to rehome a cat due to being aggressive. I had a semi-feral cat named Cirrus for about 2 years. He and Storm never really got along but I thought that in time that things would improve. Well they only got worse. Cirrus even started attacking Sky. He only liked Sunny but may have turned on him eventually too.

I learned from a foster that all of his siblings were like this. As they matured they all became much more aggressive. Luckily I was able to return him to the rescue group and they eventually found him a wonderful home. He isn't an only cat but he has a big house and a cat run outside to run in. The lady that adopted him even told me that she feels that he should be an only cat but with more space he's been doing well. I'm assuming that he's still doing well. I need to contact her again to make sure. Good luck.