Ok, I have to hard disagree here. Dean and Sam in their respective falls to darkness did not do the "same things."
Sam willingly engaged in repeated acts of cannibalism for many months. Dean did not, and if he'd done some other objectively monstrous act as part of the Mark of Cain arc, I guarantee he would have come across much stupider and less sympathetic. Sam made a series of sketchy decisions while Dean just made the one and was stuck dealing with the consequences. Sam could have cleaned himself up at any time if he really wanted to, while Dean was saddled with the Mark no matter what.
A big part of Sam's motivation was in proving himself as a better and stronger hero than Dean, but Dean had no such hang-ups about his brother or any of his loved ones, nor did the question of his strength/worthiness ever come up as a source of insecurity. Furthermore, Dean never complained about being held back by Sam nor ever labored under the delusion that he was some great, misunderstood hero. He also never once thought of the Mark as an indicator of his innate superiority. (These are all canon claims that Sam made in season 4 re: his demon blood). He didn't pursue some lofty agenda behind his brother's back, either.
So yeah, I actually agree that Dean "did it better". Not because the narrative awarded him just for the hell of it, but because his written actions/behavior/motivation actually warranted milder consequences. It's the same concept as a drunk driver hitting someone deserving more fallout than a driver whose tire blew out on a nail and swerved into someone by accident.
(On a shallower level, I was so done with Sam's endless self-absorption and navel-gazing over being a freak. Dean never disappeared up his own ass like that and only focused on dealing with his present situation. Plus, Sam's snotty, self-satisfied expression while using his powers just grated the hell out of me.)
Dean took a magical mark from a benign demon who'd been keeping to himself for centuries, had chosen to retire of his own free will, and had shown the capacity for humanity. Why would that merit world-ending consequences of the same level as Sam powering up via cannibalism, using the abilities that were given to him by his mortal enemy and apocalypse-zealot Azazel, and also trying his damndest to prove himself as the big savior of the world when literally everyone but Ruby was telling him to stop?
Sam isn't some separate entity from the story getting "sullied" by the writers if his character throughout most of the show has consistently been written to do terrible things for inadequate/unsympathetic reasons. Even Kripke's Sam in season 4 was by and large a wretched, petty, arrogant person who called his own brother weak and whiny for being traumatized by Hell (in 4.16 and 4.21, not just 4.14 under the siren's influence). I can absolutely understand hating the direction that the writers constantly took with Sam, and perhaps not giving him enough POV to better rationalize it all, but his actions and behavior as written were fully deserving of that framing, and certainly not equivalent to Dean's.
So yeah, if we tallied up the brothers' respective bad choices/actions and the motivations and circumstances behind them, Dean would absolutely come out on top. We can argue the injustice and unfairness of this apparent imbalance all we want, but I'll never be convinced that Dean and Sam's respective screw-ups were ever on the same moral standing within the written story, or that Dean's comparatively milder consequences were undeserved or proof of a double standard.