In terms of heinousness, Bellroc is the heinous standard at this point, the highest kill count also. I still do think the others still count but my vote is on Bellroc.

Usurna is the biggest hate sink of them

Bellroc and Morando are the most evil due to their resources

Usurna's the biggest hate sink, and I would honestly say Skrael, due to his unrivaled sadism and aiding in Bellroc's plans.