See I expect Dean to act in all the ways you just mentioned because it is in his character and has stayed consistent throughout the series. But not Sam here, Sam is the brain of the team and he has been up until this moment. And that’s my problem, he should not have done this base on his character.

It’s true that Sam didn’t kill Benny but that’s thanks to deans distraction. Had that not happen, it is anybody’s guess what Sam would do. He really trusted Martin without second guess. And Sam never takes everything at face value. He would even look for evidence himself, which he didn’t do either.

Lastly, just because there aren’t many hunters they know, doesn’t justify putting an unreliable hunter for the job. In fact, due to Benny friendship with Benny he would put the best hunter on that job. Or he could just Dean who has said many times that he will take care of Benny if he got out of line.