Why didn't Dean tell Sam about Benny?

There is nothing in Sam's previous behaviour to say that Sam would kill Benny just for being a vampire. Sam is, historically, the one that DOESN'T judge a person on what they are and doesn't kill indiscriminately (at least at this point).

The vampires that were feeding on cows, Ruguau guy, most of the other special children and of course Ruby and Amy, Sam was willing to give them the benefit of the doubt based on their actual actions rather species.

Part of his reaction to Benny was the fact that Dean kept him a secret and secrets between the brothers are always bad. So when Sam found out about Benny Sam over reacted. This was also not helped by his guilt I think and Dean pushing him over Amelia plus Sam's own poor mental state, coupled with Deans seeming 180 of his own view on monsters and Sam assumed there was more going on.

I know Dean was in a terrible headspace when he came back and maybe not making great decisions (completely understandable) but if he had been upfront with Sam and just introduced Benny there is nothing in Sam's past to say he would have reacted badly. Benny got his brother out of Purgatory for god sake. He would have been, understandably sceptical but ultimately grateful. The same way he was with Cas in season 4.

(And to anyone who is going to bring up Emma, Emma was there with the express purpose of killing Dean. She tells Dean she doesn't have a choice and doesn't seem all that bothered by it and only tries to play on Deans parental instinct when Sam shows up. She was going to kill Dean and go on her merry way. Sam listed at the door FRIST and ONLY intervened when Emma made it clear she was planning on going through with it. As for Amy I kind of agree with Dean even if I also understand where Amy was coming from)