Maybe at some part of the game he is the sith lord, or the Sith have separated into three different functions and are in some sort of ''alliance'' with someone else pulling the strings
A reason to go dark side. The combination Jeric mentioned can kill every single person in SW galaxy. Even Anakin. But i am talking about single person, not a wide fight
PC: Did i order you to fire?
HK: Come on master, humiliate your pett droid
Mira: What have you done!!!!!!???
HK: Shut up meatbag.
PC: Hk...
HK: I'm sorry master.
It would be interesting if you had Mira and HK47 in the same party... two assasins, one with morality(Mira) and the other with ''meatbagism'' (HK47) should have some pretty unique dialog