I had to rehome one of my kitties. I had two cats, neutered male Utu and a whole male Kuura, when I decided to get a third. I got another whole male (for breeding purpose) which was same age as my other cats, and I was prepared for the situation that the males might not get along and agreed with my best friend that she can take the other one if needed.
Well, the introduction went fine and the cats seemed to get along. The unneutered males actually had somekind of romance going on and they liked each other.

But it was the neutered male that decided that he will not tolerate this newcomer and started to hiss and growl at him. I had never heard him do that before and thought it will get better when they get used to each others.. I was wrong. Utu started attacking the new kitty, and it got worse over time.
Utu is THE cat of my life. He is my everything and there is nothing I love more than him, so the new cat was the one moving in with my friend. Most of the people who plan on breeding (or are doing it) would propably have rehomed the neutered one, but I couldn't do that.
I felt horrible, quilty and a bad cat owner.

When I went to see the new kitty, I noticed that he was a lot happier living as an only cat. He was more relaxed and not so shy anymore. Before I bought him he was living in a smallish room with lots of other males and was very shy (and later I found out that the breeder I bought him from is a liar and I don't want to be in any touch with her. I'm glad that she was only the owner of the cat and not his breeder).
He also quit marking his territory. He did it quite a lot when he lived with me (propably trying to impress my other male who he had romance with

), but not even once at my friend's place. It cleared my conscience to see that it was the right decision to let him live with my friend, he was like a different cat! I still miss him a lot, he was so sweet and handsome boy. But I know he's in good hands. I'm still his official owner and pay for his breeding related medical expences and when he gets neutered, my friend will get the ownership of the cat so I guess everyone wins here.