A Jarl's Justice

Background
Isran asks the Dragonborn to find and destroy the vampire masquerading as a Visiting Advisor to the Jarl of a radiant city, however to avoid slandering the Dawnguard's name permission must be gained from the Jarl to kill the creature or the guards will see it as murder.

Walkthrough
Speak to the Jarl and explain the situation. They will state that proof is required before any action can be taken. Locate the Visiting Advisor who can typically be found loitering in the main court. Pickpocketing his orders will reveal his true nature.

Give this note to the Jarl and they will allow the visiting advisor to be killed without intervention from the guards. Once he is dead, return to Isran to complete the quest.