"What is this Defias Brotherhood?" you ask. "The orphaned children of Stormwind," Vanessa responds. "The abandoned populace left to die in the streets. When the nobles were done with us, they cast us aside to wither away. The Defias Brotherhood is the measure of our endurance, the sign that we will not willingly tolerate the burdens of an unjust government. To that end, we have taken under our wing all the downtrodden peoples of our home of Westfall—the only home that Stormwind has allotted us—that we may rise again and collect our due from this uncaring world. And if we are to rise again, we must be willing to do deeds that may be frowned upon by the world at large but are necessary to our survival. Only through these actions and the acts of generosity that we perform for the people of Westfall will we gain our freedom and take what is owed to us by Stormwind." As she speaks, Vanessa makes another gesture to the guards. Two of them leave, then return with a bound and gagged man wearing a Stormwind tabard just as Vanessa concludes the part of her speech on "acts of generosity". "See for yourself," she continues, gesturing towards the prisoner. "You may not know this man, but he has caused us untold misery in the past. He has caused the loss of one of our most valuable members, aided those who would seek to bring us harm, and even assisted in the destruction of the Brotherhood that my father had so lovingly built. As long as we allow men like this officer of Stormwind to live, the Brotherhood of those protecting Westfall will never prosper. So if you wish to gain our trust and to take one step closer to accomplishing our mission...then kill this man."
Kill him. Refuse. See if you can make it look like you've killed him without actually taking his life. Bluff your way out of this. Pretend he's someone the Defias doesn't want to kill.