Really sorry to hear of what happened! Looked like a really nice pup. ( I call some adult dogs that.)
When my dog Lucky died, he suffered a lot. Possible cancer of the spleen, no one was really sure. He used to yelp in pain having a bowel movement. Sometimes couldn't walk. I brought him to the vet numerous time and he seemed to get better one day then bad the next. I used to just sit in the backyard with him lying next to me with his head on my lap wondering what was going to happen to him. He did pass away when we weren't home. He hid himself in the basement when he died. I heard some dogs do that, maybe not to upset thier owners? I don't know.
He died about 20 years ago but even now as I type this I still have tears in my eyes. I really do feel your pain.