Personally, I've always liked the idea that Nicodemus was the one who sold the Potter's Field to the Temple. For the uninitiated, Judas was guilty over the Crucifixion. In Matthew's account—and Matthew's account only—Judas then hangs himself. It doesn't say where. His death is mentioned in other gospels, but they're all different; one has him crushed by a chariot, and one has him disemboweled, for instance. There's a fourth that's in the Gospel of Judas, but it's considered non-canonical.
There are two conflicting accounts of what Judas did before he died. One (Matthew) says he threw the 30 pieces of silver onto the floor of the Temple, and the clergy didn't want to keep the money for themselves (they specifically called it "blood money"), so they used it to purchase Potter's Field to be a burial site for the poor. Matthew goes on to say he hanged himself, but that's all.
Anyway, we know he hanged himself, because Nicodemus has his bolero necktie. Acts says he was disemboweled, in the Potter's Field, which he actually purchased himself with the money, as an act of repentance.
Personally, in the Dresden Files, I think that both are true, but not completely; I think that Judas bought Potter's Field, then hanged himself there. Nicodemus was the person who had previously owned it, or a representative of a banking institution (one of those usurers Jesus was so uncharacteristically angry at) who handled the transaction. He got the Coins, accepted Anduriel, then spent some time walking around depositing the rest.
All of this is to say that, in my current headcanon, I'm not sure that Nicodemus was "evil" to begin with. I think he was offered power, and he took it. With the options available to him, as time passed, he started to look ahead—decades, centuries in advance. Then he started laying plans. In addition to his own personal devil on his shoulder, Nick started making questionable choices, and kept doing so, until he became the man he is today.
'Course, it's just as likely that he's the Biblical Nicodemus himself, who's mentioned a few times as another apostolic figure. There's a Gospel of Nicodemus, but it's apocryphal. No idea how any of this translates to the Dresden Files, but I figured I'd throw this out there.