I used to feel like that too when you saw some serial rapist, killer meet a peaceful end. The alternative wouldn't exactly look too good though- if they were killed in the same manner they killed with. Maybe it would be more of a deterrent but- who's going to do that to them? Another sadist?
I read a book about corporal punishment and it said that, back in the day when floggings still happened for say- stealing a loaf of bread, the general poplace began to feel more sympathy for the perpetraitors. (Obviously, they wouldn't for a serial killer.) But, it was like the powers that governed them became the bigger enemy. They starved the population to the point where they would steal and then, punished them for doing so.
I agree though. It's utterly monstrous that the most monstrous acts committed by people are punished with a (relatively) peaceful end (where the death penalty is practiced.) Yet, others who were truly good people will likely die prolongued and painful deaths against their will.