I apologize for the length. Please see the bolded areas for the Too Long: Didn't Read crowd for those who prefer.
I disagree with this some. Sam previously couldn't save Dean from hell. Castiel did, but Sam didn't resent Castiel. He was thankful, even though he found out that Castiel was telling Dean that he (Sam) was "bad" and had to be stopped. Sam had a lot of reason to worry that Dean wouldn't love him or protect him any more, because of what he (Sam) was doing, and now the angels, and especially Castiel, were giving Dean even more ideas and seemed to be actively trying to turn Dean against him. Dean even told Sam that he believed Castiel over Sam. Sam, in my opinion, could have believably pulled the "you're going to believe/choose the angels over me?" card and been jealous if he was prone to that, but he didn't and wasn't. There wasn't even a hint of it that I remember.
Similarly in season 5, Sam had every reason to think Dean wasn't going to be on his side, and again Castiel let it be known what he thought of Sam and what Sam did (and Dean never outwardly disagreed with the things Castiel said). But again Sam didn't resent Castiel or act jealous of Dean's friendship with Castiel even as their (Dean and Sam's) relationship was falling apart. Sam owned his mistakes, not blamed them on someone else*** and he didn't demand that Dean not see Castiel anymore. Again not hint of jealousy.
Even in season 7 if you looked at it a certain way, Sam had a reason to potentially be jealous. Sam was right there with Dean, but Dean was in a depression over the loss of Castiel and Bobby. Sam had a reason to potentially feel insecure, because Dean was a bit distant with him, but Sam never said anything except that he wanted to be with Dean and that he appreciated him and wanted him to get better.
I just think that if they were going to introduce jealousy as something Sam had when it came to Dean, that there were multiple opportunities for them to do so. I mean if nothing else they could've had Sam at some point admit that he had been jealous of John and how close John and Dean had been. Something. Anything, but I don't remember a hint.
As I said, it was just portrayed as so strong, so fast - like it had been brewing or always been there with all this build up and that's why Sam was all of a sudden at "kill, kill!" stage. That's what, for me, didn't ring true and made it seem much less believable.
*** And no I don't want to talk about "Fallen Idols." For me, not only was that episode subject to interpretation, it was an anomaly that didn't represent Sam's opinion in any of the episodes before or after it.
Maybe "staying gray" was the wrong way to put it.
And I did say also above that it wasn't Benny's fault but the writers' fault. I guess the best way to explain it is how a lot of people feel about Jack right now and how the writing seems to imply that maybe things aren't quite his fault.
I guess what I meant by "gray" is that Benny stayed as he was written without having the other characters - both Sam and Dean - seemingly making it much, much worse for Benny... in essence hampering Benny's progress and making it look like partially their (Sam and Dean's) fault that Benny might fail. Thereby putting some of the blame on Sam and Dean. (I remember quite a few fans on TWoP during this time putting a lot of if not all of the blame on Sam for Benny not being able to adjust.)
The writers did this in so many ways... the whole thing with Sam and Martin and taking away Benny's family support. Sam insisting Dean dump Benny. Dean actually dumping Benny. As you said, Dean was Benny's "sobriety chip," but instead of Dean being that for Benny and helping him - as would usually happen in the show with monsters who wanted to be good - Dean just said "sorry dude, I'm not gonna be there for you." (Aren't recovering addict's sponsors supposed to be there for them, not dump them?) My point being, the focus became less on Benny's story and more on how crappy Sam and Dean were being to Benny and making it impossible for Benny to adjust. In the end, they even had Dean ask a huge favor of Benny after having dumped him, as if we needed even more sympathy for Benny and more making Dean look badly.
There could have been so many different ways to do the story without making Sam "the bad guy" and Dean not looking callous, even with Benny still being good. I mean, Sam being suspicious just because Benny was a vampire and maybe things were happening that looked like they could be Benny falling off the wagon was enough for Sam and Dean to be a bit at odds, but nope... the writers had to add on and then Sam wants to kill Benny behind Dean's back, makes it so that his familial support is taken away, demands Dean dump him, Dean cuts off his support... on and on and on.
For me at that point it became less about Benny struggling and his journey and more about "poor Benny, Sam and Dean are making it impossible for him to do anything but fail." At that point rooting for Benny would be like going against Sam and Dean, because that's how the writers set it up. And I don't want to be thinking badly of Sam and Dean. I've been with them for 7 years, so a writer asking me to like another character instead, because Sam and Dean are treating him badly, didn't go over to well with me.
I get that some people aren't going to agree, and for those who already disliked Sam, they would likely care less if he was made to look badly, and some can maybe overlook Dean being made to look badly, but for me, asking me to side with Benny rather than the two main characters I have watched and cared about for 7 seasons? That's just not going to work for me.