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I don't think that we should hold Sam and Dean accountable for everyone of their actions. For the ones that are in character? Yes. But the ones that the writers threw in their for drama or cause they didn't even start to understand them? No.
Sam abandoning Dean in purgatory- NOPE, that wasn't Sam. That was fake Sam.
The Sam we know would have went absolutely feral, even more than usual because this time, he doesn't actually know what happened to his brother. He would've went crazy. He ran over dog? Okay, that'll distract him for a week or two but then he'd be back on the road. He met a woman who he likes and has a chance to be with her (and maybe start a normal life?), it'd tempt him for a solid second then he'd remember how both Kevin and Dean are missing/gone and then he'd run away. (When Dean 'All I want is my brother to be happy' Winchester finally got back from Purgatory, he would actually be pissed that Sam didn't take the opportunity to have a normal life) In season 8, Sam 'I'm actually insane' Winchester would have killed a person to save his brother like the icon he is.
Dean killing Amy after Sam asked him not to- Never happened. Why? Cause Dean 'the empath of the family' Winchester, who's been the moral compass of the show for the last three seasons, knows what it's like to have a kid and would have totally let her go. He might have gone to her house with a plan to kill her, but he would have heard her out at least. Dean actually listens and empathizes with people all the time. Why would this specific instance be any different? (Duh, for the drama, obviously)
Dean believing Gadreel at all- Hmmmmm, Dean 'never trusted any angel ever (except Cas)' Winchester would've taken the deal at first out of pure desperation because Sam was literally dying. But after Gadreel asked Dean to lie to Sam , Dean's bullshit senses would've went up. He is great at recognizing BS and he would've recognized it here.
Sam irrationally hating Benny- Do I even need to start? Sam might not be the most empathetic but he's smart enough to at least look into Benny instead of automatically declare Dean as delusion, and not listen to him, at all. He knows his brother enough to know that he wouldn't just trust someone willy nilly, especially a vampire.