Bounty Killer

 is an unmarked quest in Fallout: New Vegas.

Quick walkthrough

 * Talk to Colonel Hsu in Camp McCarran about the Fiends
 * Hear him mention Motor-Runner
 * Go to Vault 3
 * Reach the Living Quarters area
 * Speak to the injured Ranger Bryce Anders
 * Enter the Maintenance Section
 * Kill Motor-Runner and take his helmet
 * Return to Colonel Hsu for reward

Detailed walkthrough
This unmarked quest is "obtained" from Colonel Hsu, the leader of the NCR's main base, Camp McCarran. No prior interaction with McCarran is required for the Colonel to mention that the Fiends of New Vegas are causing a lot of trouble for the NCR, and this can occur by simply trying all the speech paths possible when talking to Hsu, as long as the player is not an enemy of the NCR.

Once Hsu mentions Motor-Runner (who can also be mentioned by Major Dhatri) as well as a Ranger who went missing, you can go to Vault 3 to continue the quest. Going to Vault 3 goes very well in conjunction with the Brotherhood of Steel quest Still in the Dark, as one of the parts for the air filtration system is located in the Vault. Also, the Great Khans quest Aba Daba Honeymoon requires you to deliver some Jet and Psycho to Motor-Runner in Vault 3.

At Vault 3, which is located in Fiend Territory to the west of Camp McCarran (and may be marked on the player's map by either Dhatri, the Brotherhood of Steel, the Great Khans, or the explorer perk), the player has two options upon entering. The first is to fight your way through the fiends' main headquarters, which is not an easy feat considering that as the player approaches Motor-Runner he or she will encounter numerous fiends with flamers and other heavy weapons. The second, more diplomatic, action is to pass a speech check of 65 with a fiend at the Vault door to convince her you are a Khan selling chems or simply put on Great Khans clothes and you can then enter peacefully.

After passing through the main area of the Vault, turn left and continue following signs until you reach the door marked "Living Quarters." Immediately upon entering the living quarters, take the first left down a hallway and open the last door. This should reveal a dead fiend (apparently killed by a booby trap) and a hallway with a mine and a tripwire to a grenade booby trap with one last mine at the top of the stairs. The player must go through this hallway to reach the missing ranger.
 * Note: an easy way to know you've reached the correct hallway is you may hear the screams of a dying fiends as the NPC spawns and dies
 * Note: you can avoid the grenade trap by, when the grenade is deployed, running as fast as you can down the hall, up the stairs to the left, and through the door (which leads back to the Recreational Area, so if the grenade has not exploded you will not be harmed)

Through the door is a hidden area of the entrance hall to the Vault, in which the missing ranger Bryce Anders is hiding. He will say he just needs more time to kill Motor-Runner and that you should leave. From here the player has several options:
 * Help the Ranger complete his mission by:
 * Using one of your doctor's bags to heal his broken limb then either
 * Telling him he is healthy enough to help you kill Motor-Runner
 * Convincing him he is too hurt to complete his mission and telling him that you will kill Motor-Runner yourself, which will send the Ranger back to Camp McCarran but will not complete the quest
 * You can also betray the Ranger by
 * Immediately killing him, or
 * Convincing him that he is healthy enough to fight Motor-Runner alone (this must be done BEFORE giving him the doctor's bag) which will kill him and grant you negative karma

If Anders is convinced to leave and healed by the bag he will open a passage from the room he was in directly to the Vault 3 entrance chamber and sneak out of the Vault. This allows you to go directly from talking to him to the Maintenance Room of the Vault, which is relatively easy to find (just follow where the heavy troops are) and is where Motor-Runner is located. There are several ways to kill Motor-Runner: Basically, if you passed the speech check at the beginning you should send the Ranger on his way and kill Motor-Runner yourself with no resistance from the fiends, and if you are fighting your way through you should take the Ranger with you to fight your way through the vault and kill Motor-Runner. The latter grants a slight bit more EXP. (52xp as opposed to 50xp for doing it by yourself)
 * Battling your way past fiends with flamers and heavy weapons into Motor-Runner's throne room, which houses him and his dogs Bone-Gnash and Ghash-Bone. This is by far the most inconvenient way to complete the quest and is the only way to kill him if the Ranger is with you, as the entire Vault becomes hostile
 * Telling the Ranger to leave the Vault, then (as long as you passed the speech check at the beginning) simply applying a stealth boy, hiding in a corner of the throne room, and killing Motor-Runner with either a silenced or melee weapon (or a regular weapon, but this will turn the Vault hostile unless you are very well hidden). Alternately, you can pickpocket Motor-Runner and plant a C-4 charge on him. Detonate it while hidden and the Fiends will not turn hostile.
 * If you have already completed Aba Daba Honeymoon, the fiends may remain non-hostile to you, and it is possible to stealth kill them with silenced weapons in and out of VATS without them turning on you. This even works in groups of them, as long as you are hidden when you attack.

If you retrieve Motor-Runner's helmet and deliver it to Colonel Hsu, he will grant you NCR fame as well as caps.

Journal entries
None, this is an unmarked quest.

Bugs

 * If you proceed straight to Motor-Runner after passing the initial speech challenge and kill Motor-Runner while hidden, you will be unable to find Anders, who is gone when the Courier reaches him. He will reappear at Camp McCarran where he will initiate dialogue involving Motor-Runner as if in Vault 3. More info on this glitch in the bugs section here.
 * If you fight your way through the vault and kill Motor-Runner, and then backtrack to free Anders, the panel will be open and he will already have left. When you report back to Colonel Hsu, he will tell you that Anders arrived back at the base a little while ago, and thank you for freeing him. (This may also depend on having fought your way into Vault 3 in the first place, in the process killing all Fiends in the area.)
 * When you claim the bounty on Motor-Runner by bringing Colonel James Hsu the Fiend's helmet as proof of the deed, the helmet does not get removed from the inventory, and cannot be dropped (since it is a quest item), although it does not retain its 1.00 weight. It can be removed with the console command.