Most realistically good character in fiction

[Quickman]

Logan Ninefingers from Joe Abercrombie's "First Law Trilogy".

I see what you did there.

Mmm, no he isn't. He's The Atoner. (Careful, tvtropes.org link.) He tries, and mostly fails, to overcome his evils. Trying to be good is a major step to being good, yes, but in the end he is still the Bloody-Nine, and being a terrifying merciless berserker is the only way he can ever achieve what he wants to see happen; he isn't capable either morally or mentally of doing better. He is a tragic hero, and tragic heroes fail because of tragic flaws.

If you want a believable fictional good guy, try Michael Connelly's police detective Harry Bosch, of his long-running series that begins with The Black Echo. They are mostly excellent books, too.

...however, he is of higher morality than those he fights. Even if we do believe he is " more grey" he is ultimately the lesser evil than those he opposes. Conan the Barbarian did it for the sake of reading and plundering the land. Roland Deschain wanted to see the Dark tower and climb to the top.

Logan however, is moral.

Your argument states that "killing is bad" without observing the fact that he is the lesser evil and using the techniques he learned through warfare, prowess, and fighting as a way to defend others later on (even make the more moral of the two outcomes to happen) and ultimately reach towards the greater goal.

Edit - also TVTropes are the written, subjective opinions of others. Take it with a grain of salt.