A thought I had while going over the Faces of Fear recording I did earlier today.
Why do each of the Faces of Fear act the way they do? Saren hears voices in her dreams that tell her to kill and mutilate people. Warrick is shunned, humiliated, and constantly mistreated by the woman he loves and follows her bidding in an attempt to win her affection. Radok is much like Warrick in that he idolizes Garrett and seeks to win his approval and recognition. But Garrett? He just likes killing people.
Garrett is not like a serial killer; he does not pick out a victim, fantasize about killing him, stalk him, and finally do the deed. He just likes getting into fights and killing people. That is why he became a soldier. That is why he became a mercenary. That is why he became an assassin. And the fact that sometimes his patrons did not want him to kill certain people is why he left even those jobs behind to join Saren's cult. Here, he basically has free reign to kill whomever he wishes (fortunately, he feels an odd, familial love for Saren and all but ignores Warrick, so he has no desire to kill his fellow Faces; I still do not know why he lets Radok live; probably likes to see the little guy struggle).