Oh katydid, I know how hard that must have been. In November I had to put my 16 year old cat down. She was attacked quite ferociously at times by one of the other fur babies. She actually lived in the bathroom for awhile because she was so scared. She did get sick from a stress disorder which lead to very invasive treatments by my husband and I. She finally let me know she had had enough. I could not get her to stop vomiting even with the vets help and she went into liver failure. She was an incredibly sweet, soul and would not protect herself from the other cats.
Please know you did absolutely the right thing for her. I know how much it hurts. I am still missing her so much everyday but it gets a little less awful with time. sometimes there are no good solutions. You just have to do the least bad one.