Arizona Killer: The Courier will be required to meet with Cato Hostilius between Ranger station Delta and the cazador nest on top of the mountain. There, he will offer the Courier advice on how to assassinate President Aaron Kimball before resting. Upon waking, he will be dressed in civilian clothing, give the Courier NCR armor as a disguise, and rush over to Hoover Dam in the middle of the crowd awaiting the President's arrival. After the quest is completed, he will disappear from the game.
Killing Cato during the quest strangely doesn't give you negative reputation with Caesar's Legion, but it will cause you to fail the quest.
If you have an Explosives skill of 50 you can get Cato to give you a Vertibird bomb, which you can use to kill Kimball. If you don't have the required Explosives skill, you can pickpocket the bomb off him.