Initially, the default player character Tav was the Bhaalspawn, who started this whole plan with Gortash and Ketheric, but Orin backstabbed you and took your place. That's why they know you, and "trust you", as in trust that you would participate.
Players didn't want to be evil, and didn't want this villainy which they felt was forced on them, and it was set aside during development. You can see it in Swen's interviews, years ago he was proclaiming how you get to be evil, you get to be the bad guy, a lot more often than he was closer to release as it was toned down for mass appeal.