Hace El Oso
Arcane
I don't think looking at them as a proper "race" like for example the Drow is the right way to approach them. Racially they are humans, but they are tainted with the blood of Devils, which ARE a universal force of evil. Despite how any individual Tiefling chooses to behave they are still ultimately a way for Hell to defy universal law and gain a foothold on the prime material plane.
I don’t think that there is any way that can be made to work with the modern sensibilities of the usual suspects. It’s funny, it used to be that some people actually enjoyed roleplaying as an evil character (and everyone else enjoyed fighting them), but as all of the ‘talk’ has become more and more about promoting ‘moral grey’ the more people expect everyone to have the character of an ingénue or a maiden aunt.