Years ago, I faced the heartbreaking choice of how to remove one of my three kitties from my household.
I brought Saber into my home when her owners, my neighbours, moved away and were going to just lock her out of their house and let her survive on the street.
My already-resident cats LOATHED her and made her life miserable. I suspect that when I wasn't home, Saber literally did not move from her hiding space under my bed until I returned hours later; she was that tormented by the other 2.
Finally, I accepted that Saber was living a miserable life and that she'd be better off out of my household. After months of trying, I was unsuccessful in finding anyone who would adopt her (she had the annoying habit of pissing on everything I owned, sometimes even while I was using or wearing it!), and the local humane society puts down kitties like it's going out of style.
One night as I tried to find her a new home, I had a horrible nightmare. In it, I visited a pet store that I used to work near. In the dream, I returned to that store for the first time in 5 years, and found my poor Saber in one of their cages! She'd been locked up the whole time (ie, 5 years), with no family or outside playtime. It was horrible, I felt so fucking guilty that my tears woke me up.
Soon after, I decided to put Saber to sleep rather than risk her being in a cage for the rest of her life. Imo, she was better off that way; in her last moments, I held and stroked her while they gave her the injection, and told her she was a good girl and how much I loved her.
Some of you will probably freak out on me for this, putting down a probably-healthy cat, but I couldn't find any other options that wouldn't have also resulted in her death or worse.
Anyway, back on topic, about once a year, I again dream that I find Saber has been locked up in a pet store for years. It's always a very unpleasant dream and brings back all the horrible memories of her death. I guess it's guilt over my choice, even though I still stand by it.