I tried an evil character, but it seems like this game severely handicaps you if you try an evil playthrough. My first attempt was to string along the ncr and do most of the mundane missions that didn't really effect the final battle and were more merc like. I didn't kill any fiends, and helped the powdergangers in goodsprings, and ncrcf. I managed to maintain a goodsprings rep that allowed me to not be shot at and still trade with chet. My biggest mistake was going to nelson, and killing the crucified troops for the ranger on the hill. I figured I could snipe them silenced and get away with it, but somehow the legion in that town magically knew it was me even though I remained hidden, and shot me on sight after that even though all the other legion camps still were neutral. The only reason I did that mission was to bring up my rep with ncr just enough to keep the ranger kill squad off my back. That locked me out of that mission. I suppose I could just go psycho and kill all ncr troops I see, like I did with the legion troops on a previous playthrough, but I wanted to play a sort of double agent, with a lot of mr.sandman, and stealth kills. Didn't pan out well, and seems to be much more trouble than it was worth. If I take the psyco approach my xp pool seems very limited. I did the kings quests and stirred up the trouble between them and ncr, and the van graffs, the kahns, the brotherhoods, even though I plan on killing them all before the end, but otherwise, I guess I have to just go cave hopping to make up for all of the missed ncr xp. FO3 was much more fun to play evil, and you had tenpenny, slavers, and even evil followers. I'm a little dissapointed.

Hmmm... I'm kind of doing double agent good guy at the moment... stringing along the Legion until I break and just turn into a Legion hunting and killing machine. I was wondering how this would play out evil (aligned with Legion) given they have no presence in Vegas. I'm going to make it even harder and play it in hardcoe mode only using melee and unarmed combat... maybe explosives just to use up the skill points.... although there aren't exactly an abundance of grenades in the game either...

I killed Boone and all the NCR in Camp Forlorn Hope, Camp McCarran, all the Ranger Stations, and still the NCR keeps changing from "Vilified" back to "Merciful Thug" or "Wild Rascal" or "Shunned" and they all still talk to me. At first I had to wear an NCR disguise to approach them, but then I realized they would still talk to me even without the disguise. Some of the NCR quests would only be given to me if I was wearing the disguise, however.

In fact, after doing several NCR quests, I was Idolized by both Caesar's Legion and the NCR at the same time. Now I'm back to Wild Rascal with NCR after setting the HELIOS laser to kill them.

Once, in order to fix a corrupt auto save file, I killed the Ambassador just prior to blowing myself up and that seemed to initiate the Don't Tread on the Bear quest. I think once that quest is initiated, you can't go back to the NCR side, or it may be very difficult to do so.

If that is your situation, you might try a console command to change the status of the Don't Tread on the Bear quest.

EDIT: so far after 160 hrs, I still have not set foot inside the Lucky 38. I have heard that visiting the Lucky 38 and talking with House resets your status with NCR to neutral.

Good thing to know is that facing Benny and giving Mr. House his chip will reset your reputation with NCR or Legion back to Neutral if it was worse (this gives you NCR Ambassador and Caesar invitation).

So you can, at start, be fully anti-legion (take Nelson for NCR and stuff), and then later switch to being buddies.


P.S.
Don't Tread on the Bear is just warning and doesn't affect anything. Only failing that quest kills NCR questline.

There are a lot of missions that are mutually exclusive. Obviously nelson is one. And cottonwood cove another. Otherwise I haven't just gone to the fort looking for work yet. I am wondering if I should just do the main story up to killing benny for the chip and get the invite to both camps, or wait to do that just to get the slate wiped clean after I clean house terminator style.(maybe a visit from mr.sandman first) I plan on doing the BOS missions until I get training, then kill them and take their stuff, but I'm more inclined to kill them all in their sleep 1 by 1 until the end to keep the vendor and doctor.