Did you refuse to get George the Drudanae herb at the start of the game? At least in the unpatched game that will break the murder quest, even if you later bring him his fix.
Did you go through all the speech options with the vampire before attacking? You need him to confess to the murder, and reveal the reason behind it.
A couple people who had problems reporting back to Mardanius were able to do so after he moved to the council hall, later in the game.
Quests do not all have the same experience rewards at the same character level.