Uh, I guess I'll be clearer so you don't actually have to read the whole Dexter book:
Dr. Danco was a US "advisor" in El Salvador. He was a medical school student who found he was a sociopath, but he had a deep patriotism and really wanted to use that. So they had him torture prisoners. He would slowly slice them apart, bit by bit, taking off fingers, toes, hands, feet, legs, etc. All in front of a mirror, all with the "patient" kept alive and awake with a crazy mix of stimulants, pain killers (the pain would kill someone without them) and hallucinogens. He would talk to them throughout the procedure, asking them about their personal life and such. He'd slowly cut away everything extraneous. Both legs, both arms, all genitals, ears, nose, lips, tongues, teeth, and eyelids. Leaving them a screaming lump in front of a mirror, unable to look away from their image.
Eventually he got a reputation and the US decided that it would be best to turn him over to the other side (who would of course kill him) to create good will, but he was taken by the Cubans who put him in a hard ass prison and then used him as a torturer. Eventually he comes back and starts going after the people who tried to have him killed. He plays a game of hangman with each one, each wrong letter means losing a body part (although the speed of dissection varies from victim to victim). No one finishes a word.
Anyway, something like that, someone clearly fucked up but with noble intentions, at first. Then they are betrayed and are seeking revenge in a REALLY awful way. That method of killing someone (leaving them a living, absolutely insane, completely hopeless lump of flesh) is about the most horrible I can think of.