[DF] Why not have some pure "bad guy" races in DF?

[Lonewulf]

Trolls were actually complicated in Scandinavian folklore; they were a bit more complicated than "likes to eat people".

http://en.wikipedia.org/wiki/Trolls

The Vitterfolk would capture humans to use as slaves, or at least as prisoners amongst them; that's about the worst you can expect (as well as steal your food and spoil your beer!) But they're a bit more complicated than you're making them out to be.

As for the ultimate answer to why no "pure" evil monsters? I think it's because society is moving away from that kind of perspective as a whole. To be honest, the idea of a race being "innately" evil, and their children being "innately" evil (would you kill a Hobgoblin kid? If so, you're already touching on what makes the concept disturbing), is rather reminiscent of the ideas of a race being "superior" to another, or another race being "inferior", thanks to genetics.

Also, it's fun to make the evil monsters look, from their own perspective, to actually be rather sympathetic and understandable. It also, I would argue, makes for better (or at least more in-depth and complicated) storytelling. It's easy to invent a bogeyman; it's harder to make the bogeyman likable.