I looked up the unique GUNS on the NV wiki page and decided to collect a bunch of them, I have All American, Gobi Campaign Scout Rifle, Big Boomer, Dinner Bell, Ranger Sequoia, The Ratslayer, La Longue Carabine, and the Mysterious Magnum. Now, to get some of these I had to kill these people: Chief Hanlon, Corporal Sterling, Old Lady Gibson, and The Lonesome Drifter. The first two were NCR characters, but I took them out with a silenced sniper rifle so that I wouldn't get any infamy with the NCR, but I failed a quest after killing each one, four in all. Will killing these people do anything to the ending? Have I ruined my character by doing this?

Edit:

Chief Hanlon - Location: Camp Golf - Quest Failed: Return to Sender

Corporal Sterling - Location: Camp McCarran - Quest Failed: I Don't Hurt Anyone

Old Lady Gibson - Location: Gibson Scrapyard - Quest Failed: Come Fly With Me

The Lonesome Drifter - Location: Near El Dorado Gas & Service (by a billboard after you activate the Talent Pool quest) - Quest Failed: Talent Pool

You could have gotten Chief Hanlon's and Lonesome Drifter's guns without having to murder them, simply by completing relevant quests. Return to Sender actually earns you an achievement.

Old Lady Gibson plays a role in a number of quests, but Come Fly with Me is one of the game's best quests, and also earns you an achievement. Plus she's a useful merchant selling weapon mods.

Killing Corporal Sterling doesn't fail I Don't Hurt Anymore unless the rest of the 1st Recon turn hostile. It does fail Old School Ghoul, which allows you to upgrade Raul.

So no worries then, I always bypass Come Fly With Me as on the white Xbox 360 without a hard drive or updates, and NV unpatched, that quest causes a savegame glitch that doesn't allow you to load your save. I also don't have Xbox Live so I don't worry about achievements or gamer points. I was just worried that I might have screwed up the story line.

There's no need to kill these people, all you have to do is steal the ammo from them, leave the are ( ie; fast travel to a far location), go back and they will have put weapon into inventory, just steal it from them (it may take a few tries, especially with the lonesome drifter, for some odd reason its hard to steal).

I do it all the time, again, there is no need to kill them, plus old lady gibson sells weapon upgrades and has lots of bottle caps that refresh every few days.