The Karmic scale fits nicely in Fallout 3, how was this concept developed?

Karma has always been a long standing part of fallout. Given Fallout 1's roots as being developed by a group of Classic RPG fans, I would imagine it probably grew out of the alignment system in Dungeons and Dragons.

Although that said, DnD's scale is a bit more of a "Select a Straightjacket to play" as opposed to Fallout's feedback driven sliding scale.

Not DnD.
Pen and paper rpg: GURPS

It's true that Fallout's SPECIAL game system is sort of relative to GURPS. By being quickly bashed together replacement for GURPS after SJ-Games withdrew license. Still, GURPS doesn't have karma nor alignment system, there is far more specific disadvantages that effect behavior of character in some ways, Code of Honor disadvantage is quite good example. Some GURPS settings/expansions might have something like karma or alignment, but vanilla game system doesn't have that.

As before GURPS has no alignment system. Although GURPS was intended on being the original ruleset, its removal from the game is one thats unclear (Interplay blame SJ, SJ say interplay stopped talking to them and they never pulled it.... Given SJ's openess about pretty much everything, I'm more likely to believe them. IIRC they even published the emails at one point).