You're right that the dark themes in Korra were given more attention than the ones in ATLA; that's absolutely true. Do consider, though, that LOK was not aimed at the same demographic that ATLA was. LOK was aimed at the demographic that had watched ATLA originally and had aged considerably since then. I'm guessing Bryke felt more comfortable depicting a harsher (and in some ways, more true; we do, after all, see ATLA through a more child-like pair of eyes) picture of the world because of this.



I would contest your characterization though that LOK "relished" pain and suffering. I only agree insofar as they were both prominent themes throughout the series, but certainly life has its fair share of both, so I don't feel depicting them in an honest fashion can be equated to enjoying, or relishing, said depiction.