A non-cliché ending for a killer

Bad guy learns from terrible guys, has rough time, hires own gang. Makes a mediocre name for himself. Then spends long time trying to make the perfect score. Becoming more and more sick of the life. Longing for peace. Still extremely good at his craft he plods along. His ability to handle a large gang grows with him as he ages. Then finally a bold plot. The perfect score. He hired a huge band of gangsters. Then used them like an army to make the heist. Then the remaining gang he tricks into a mass grave. No trace of him nor the prize was ever found. All to were erased, all the people dead. Each day the townfolk commented on how the young couple took in her aging father whom seemed to be adrift in his own world.
Just him his grand daughter his daughter and his son in law. None of them ever knew who he really was. He lived another twenty years in absolute peace. Just him his beloved family, and his ghosts. Dying at the age of seventy five. No one ever learned his secret.
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Let's see, the main bad guy in my first book had his throat cut, decapitated and his head stuck on a pike. In the second, the bad guy fell through the floor of his burned-out headquarters and into a molten pool of his own treasure (mostly silver). And in the third . . . not published yet, so I'm not telling.
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I am a fan of fantasy sooo... turned into a Chihuahua. Can't speak, can't really communicate effectively. His sanity and memories slowly slip away forever, to be replaced by the dog's. He is then taken to the vet's, neutered, and surrendered to an animal shelter. Within a week/two weeks, he never remembers that he was ever human. He is adopted by a loving human family, but is never discovered to have once been human, and thus is never restored to human form. He lives out the rest of his days as a dog... which is extremely short, because a 20 year old human is a lot younger than a 20 year old dog...

Hoo boy that was an ugly side of darkness, but that's what I've got.