I don't think anyone's getting a pass. There's just so many choices throughout the biblical narrative. I imagine if you could ask Uriah he'd say David was the most evil person.
It is certainly a paradox that David, a man after God's own heart, would commit adultery and murder in order to achieve it. There are many, many lost people that never did that. The way we look at things is certainly different than the Lord. Another person that comes to mind as the most evil, above all the OT characters is Caiaphas, the high priest at the crucifixion. Yes, there were many, many OT evil kings of Israel, Judah, and other nations. However, they were openly evil and everyone knew exactly what to expect from them. The Pharisees on the other hand, were pretending to be something they were not, the keeper of God's law. If one thinks about how the Lord would answer this question, I think old Caiaphas would be at or near the top.
Christ Himself, who is God, gives us a hint. He got visibly angry at two situations, making a buck off the house of God, and the hypocrisy of the Pharisees. Terms like "whitewashed tombs" and "brood of vipers" is a description Christ saved for this group alone. If one really gets into the verses that record the exchange of words between Jesus and the religious leaders of the day, one can feel the sparks flying. It is the most confrontational Christ got with anyone, more than the prostitute, tax collectors, and the lower class. Even Pilate, it is clear the Lord felt pity for, although Christ did let Pilate know exactly where his power came from.
I would say from the Lord's point of view, just going by Christ's words, the Pharisees are way up there on the most evil list.