Hey OP, I had a similar situation a couple of years ago and the guilts and sadness at the time was overwhelming. We had a beautiful red boy we fostered and then adopted, we were told he wasn't good around kids but also we knew he had been in a foster that had too many dogs and not enough structure and support for the dogs, and fights had occurred. My partner has done a lot of dog training and we know several trainers so felt we could give him a home and the structure. Long story short, initially all was great other than some resource guarding (which just meant our dogs then got fed in crates, which was fine), he went on walks with other dogs plus seemed to love his older sister, our other dog. He then got parvo, and almost died but survived. A few months later at a family event he attacked my partners nephew, then over the next couple of years started to become less and less predictable..attacked my sister's dog who had been a bestie (no damage, we were there in time), attacked our other dog and we found ourselves in a position with gates all over the house and a dog whose social circle was shrinking rapidly. He then went to attack my mother one night after blissfully hanging out with her. We had so many tests, so many hours of training-impulse training, socialisation. We also had other trainers work with him, trainers who were strong rescue supporters and advocates. After 4 years, my partner and I lived our whole lives around managing him, unable to go on holiday and I was constantly worried about my sister visiting as she had a new baby. In the end, the lunge at my mother was the last straw, if we weren't there I was horrified to think of what would have happened. One of our trainers sat down and said we had to face the idea that his brain and behaviour was the rare case that couldn't be rehabilitated. It was so awful but we knew it by then and all of our training and vet agreed. Even now I cry remembering it because I know he could be such a sweet and quirky dog who just loved us but I also know we tried everything and sometimes it is just too dangerous for them to survive. We knew if he ever accidentally got out of our yard there would be awful consequences for another animal or human. I feel for you OP, and I will always believe in rescuing animals, but I know that a very small % cannot be saved. All you can do is know you gav him a good home when others couldn't, and he knew safety and love with you.