I'm trying to increase my Karma by killing Powder Gangers, but I'm not getting Karma increases for it.

Is there a bug?

You wearing a faction armor?
I dunno if Powder Gangers are "very evil" though, they might just be set as "evil", and in that case you shouldn't get karma increase.


[edit]

Besides, Karma is useless in New Vegas for the most part, so why even care?