I'm not convinced Ezekiel is benevolent. It looks as if the angels are each searching desperately for "strong" vessels (of which Castiel's body and Sam's body each qualify), and Ezekiel manipulated Dean and Sam in order to gain possession of such of a strong body. Ezekiel isn't healed enough to keep from being forced out, so he convinced Dean to keep quiet until he is healed enough to make the possession permanent.
It may also be that Ezekiel is working for Metatron and will use Sam's body to finish the trials. Though, I'm less keen on that idea.
Side note: did anybody else notice the creepy smile on Ezekiel's face when he took possession of Sam? I don't think I was imagining it.
As for Castiel: he's not fully human. He can still hear the angels in his head.