We had the same problem with a cocker we rescued. We had lost Danny, our ESS on 11th May and picked Brodie up on 12th May. He was fine when we first got him, but over the 5 weeks we had him his behaviour became more and more unpredictable and we had to return him.
A behaviourist checked him over and was attacked twice and on the vets recommendation he was pts.
I know its a subject that many feel does not exist, but they said he had rage. It was like a switch was turned on and he became another dog, but when it went off he was fine again. I was scared of him at times and I am sure he realised this, but he never had us locked in or out of rooms. I have had had 7 spaniels and Brodie is the only one I have had a problem with, but when he was good he was very good, but 1% of the time he scared me and snapped at me, my wife and my son. He also fought with our other dog, Minnie and savagely attacked Jess, my daughters cocker more than once. He seemed to be good with my 4 year old granddaughter, but I could not risk him turning on her and maiming her for life.
Although we were distraught when we found out what had happened to him, at least he is now at peace and he never actually bit and maimed anybody.
Its 8 weeks since I took him back and 3 weeks since I found out he had been PTS, but I still think about him every day and wonder ..... what if?.........
I cried every day until I found out he had been PTS, but even though that hurt like hell, it gave me closure, but it resulted in me having to prescribed anti depressants to help me lead some sort of normal life again.
I am a lot better now, but the GP feels I'll need to stay on medication for some time yet and last night my wife said she feels I am getting back to my old self again, but believe me, if I could have done anything to have saved Brodie I would