'Ubi solitudinem facuint pacem appellant.'
Pure, unadulterated evil is rare and so are the Baali. They tempt others to let their dark side free, to just give in. After all, it is so much easier...
The Baali serve the darkest force that exists in the Realms, Ba'al. Be it through devotion to Vl'aresch, Mak'kor, LaChte, or Bael, in the end, the Greater Demon Lord is served.
They are not, however, senseless killers. Rather, the Baali are the gentle corrupters who tempt all of mankind. They are the serpent in the garden, the dark voice that whispers from the back of one's mind. Your eventual corruption, their "raison d'etre".