And another thing . . .
Sam?
You remember the time when your visions freaked Dean out? When Dean had people telling him his brother was a monster and needed to die? When you yourself were infected with what everyone thought was a virus that was going to make you kill crazy? Did Dean do what any other hunter might have done and gank you? Heck, at one point you (well, really Meg, but Dean thought it was you) went on a killing spree, then begged Dean to kill you. But he didn't. He had faith that this wasn't you. That you could be saved. "If you can't save Sam, you'll have to kill him," and Dean's philosophy moving forward was, "I'm going to save Sam if it's the last thing I do."
Now, why the heck wouldn't he give his brother-in-arms the same benefit of the doubt? Benny got Dean out of Purgatory. He has also given Dean no reason to believe that he has not done a Lenore (remember her? The vamp you talked Dean out of killing?)--if not exactly going "vegetarian" then at least drinking blood willingly given. And you want your brother to turn around and gank him? Is that the kind of loyalty you've observed in Dean his whole life?
You might be right. Benny might be blowing smoke, and you might have to kill him. But going by past experience? Dean's judgement is pretty good when it comes to working with monsters. He usually knows going in what he's doing, that things could go very sideways very quickly. But this has not been the case here.
Methinks perhaps your year with Amelia has fuzzied your memory of your brother. (You know I still don't trust her, right?)