Basically, Knight Templar wants Star Trek to feature the kind of diverse personalities that would realistically never make it past the presumably thorough psych evaluations that Starfleet undoubtedly requires all of its cadets/personnel. If someone's motivations for exploring the galaxy are rooted in getting revenge, or fulfilling some fanatical quest, or anything else so psychotic as what he's suggested as possible motivations, one would hope that they'd be culled from the program prior to graduating from the Academy, and would never, ever make it so far as a shipboard posting.
Not at all.
You could have someone in Starfleet already who has fundamentally changed to darker as a result of their experiences there.
Experience in war such as the loss of a ship perhaps.
Or someone who has after 30 years in the service or so has decided that he "really does not like the damned aliens much"