Yep, I was actually shocked when I played my cannibal legion playthrough. I ate every corpse, murdered every1 at HELIOS one, Helped the Powder Gangers at the NCRCF, helped them take good springs, killed every prospecter I came across, murdered the traders at the grub 'n gulp, stole pretty much everything that wasn't nailed down and finally send the ghouls to a fiery death in their space ships by convincing the human looking ghoul to sabotage the project. I also deliverd Cass to Jean Baptiste and then went to mr. House. I thought let's check my Karma: "Protector of the Wastes" WTF?! At that point I actually stopped being evil cuz... what's the point?
Interesting you mention that, my current character is a pro-NCR technophile with cannibalistic tendencies who's also rotten to the core. Lots of corpse eating and stealing, and her karma has been at Very Evil almost the entire game. Then again I have been avoiding certain quest conclusions that increase karma, as this character is self-centered rather than the altruists I usually play, which has helped quite a bit as some of same are quite substantial.
It
has required quite a bit of effort, especially compared to how I usually hit Very Good without even trying by the time I'm level 8, and she did rise to merely Evil when clearing out the Fiends blocking her path to Vault 3, but eating the resultant corpses got some of it back and the rest came when she stole everything not nailed down from Cottonwood Cove. As she's somewhat unstable she will eventually turn on the NCR (I planned ahead of time to go Independent), and then keeping max negative Karma will be fairly easy.
One thing I haven't done, which would probably help you out if you can manage it without getting ventilated in the process, is to steal everything littering the McCarran concourse area; at -5 per item, there has to be at least -400 Karma in the cafeteria alone, and that's only one portion of the cell. I do agree that the +karma for killing Fiends is grossly exaggerated, which is why I toned it down in my game; she'd still be at Very Evil right now, but would have risen to Neutral in the interim which would have been slightly annoying.