Dean could feel the tension building up in Sam bubbling just below the surface like a volcano ready to erupt at a moment notice and he himself was not helping with his closed off I don't share my feelings attitude. He'd experienced a precurser of his brother's anger & frustration when he had finally broken his vow to his father & told Sam the truth, or rather the truth as told by John Winchester whose orders were always on a need to know basis.
He would have normally laughed at Sam's comment that he could go "darkside" if it hadn't cut so close to the bone. Dean had quipped before that Sam was Luke Skywalker and he himself was Han Solo (the cocky good looking one who always got to "drive" and shoot the bad guys) but now he felt he was losing Sam and rather than him being the innocent Luke who grew up to be a hero he could possibly be more Anakin Skywalker who lost his mother, his girl & finally his humanity in becoming the personifation of evil Darth Vader.
Sam had left him briefly after his father's revelation had surfaced and although Dean understood that Sam needed time to process this dark secret he couldn't bear it that he was unable to protect his brother wherever he had crept off to. It nearly broke Dean when he finally caught up with Sam only to believe his brother had unwittingly died in Gordon's trap & he had been the bait to lure Sam to his death.
They would both have to face it sometime that Sam could go bad. Hell, Sam had already faced it himself it was just Dean that was still in denial. For Dean the fact that his Sammy could go "darkside" was NOT an option, not while he was alive anyway. He had faced down two Reapers in less than a year so death was not something he feared, it was something that was always inevitable in their line of work.
Dean Winchester feared nothing, no that was a lie, he feared only one thing. That one day he would look into the eyes of his beloved brother to see only black soul-less pits staring back at him and that would mean the end of the world to him