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 the Motor-Runner

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.

One Hsu mentions the 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.

At Vault 3, which is located in Fiend Territory to the east of Camp McCarran (and may be marked on the player's map by Dhatri or by the Brotherhood of Steel), 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 a Khan selling chems and you can 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 killed by a booby trap and a hallway with a grenade booby trap. 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 is 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 himself, 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 the 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 the Motor-Runner is located. There are several ways to kill the 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 much more XP.
 * Battling your way past fiends with flamers and heavy weapons into the 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)

Bugs
The most prominent bug in this quest is the error that may occur when telling Bryce Anders to leave the Vault, and that you will kill the Motor-Runner yourself. It is possible that he may glitch and forget to sneak when leaving; he will be attacked and killed by fiends. If that happens, reload and choose the other answer at the end of the dialogue ("I didn't come here to do your job for you"). Anders will still give you the Vault 3 meintenance key, only with a different comment, and then he'll sneak out.

Additionally, Anders may be killed while making his way back from Vault 3 to Camp McCarran, which would not really change the outcome of the quest but is rather sad.

Journal entries
None, this is an unmarked quest.