After all that crap he went through, he isn't able to make it through it all.
At least he no longer has to suffer.
I know a dog doesn't compare to an uncle, but when my dog was dying of cancer, he was in so much pain and suffering even with the radiation treatments that I felt it was better to put him to sleep rather than have him endure so much agony. Too bad he died on his own, in pain no doubt.