In fact, Dean never explicitly says that Benny isn't killing. He compares Benny to Kate the werewolf, and Sam figures it out: "She was different. She – you think Benny's different? He tell you he's not drinking live blood, or something? And you believe him. Wow. Okay. You know, you're right. People do change."
Dean does not express much faith that Benny will refrain from killing. A little later, he says, "Look, Benny slips up and some other hunter turns his lights out, so be it." Implication: He won't be able to do it himself.
It's actually striking the Dean is so honest. If he had insisted he had faith in Benny's ability to stay clean, Sam might have accepted that. Instead, he doesn't pretend to be confident. He doesn't hide his serious doubts.