Report Ser Uthor’s Statements To Queen Cersei And Put A Stop To His Slander.
Objective Point A
The best way to finish this quest is to threaten Uthor first. When the queen sends you back to Uthor, chose to explain the situation to him and there will be no fight. The queen will be extremely pleased with your actions.
Otherwise, after you speak with Queen Cersei in the Red Keep’s throne room, look for Ser Uthor Donnerly on the right side of the red carpet as you exit. He believes the queen has murdered his former master, Jon Arryn, and won’t keep quiet about his distaste for the queen and her policies.
Objective Point B
Leave Ser Donnerly and return to the queen. Report Uthor’s statements and listen to what the queen has to say. She doesn’t want to make a scene and won’t be happy if you kill the knight, but she does agree to let you handle the matter.
Return to Ser Donnerly. He doesn’t want to listen to what you have to say, and he has a quick temper, so you may end up battling him anyway. Even if you can’t convince him to keep quiet and it comes to blows, you will complete the quest despite the queen’s disappointment.