I do not own Supernatural
Hero: a: a mythological or legendary figure often of divine descent endowed with great strength or ability b: an illustrious warrior c: a man admired for his achievements and noble qualities d: one that shows great courage.
-Merriam-Webster
Sam wasn't a mythological creature; he was, for all intents and purposes, human. All his abnormal abilities hadn't come from anything divine; in fact, they had come from something much darker in nature.
Illustrious? Try infamous. Most hunters were warriors in their own right, and Sam was well known in the hunter's circles, but people don't really think good things of the guy who jump started the Apocalypse. No one was admiring him for his achievements and discussing his nobleness over a cold one. As for the courage thing, well, no one had ever accused Sam of being a coward, but you didn't have to always do the right thing to be brave.
Nope, the general public thought Sam was a psychopath serial killer who was dead. The few in the know thought Sam was a traitor to humanity and no better than the devil himself, and the few that had truly known Sam, well, even they knew he had a darkness in him.
But did that mean Sam was something other than a hero?
Not in Dean's eyes.