Garrus reminds me a little of Batman and the Punisher in method but totally not in attitude. Batman's almost philosophical about it all and tries to keep it form getting personal (with limited success). I kind of get the feeling the Punisher is more than a little bit in it for the revenge. Garrus is a creature of fierce passions. It's like he finds the very existence of evil baffling and frustrating and doesn't understand how it can possibly be allowed to exist. A good deal of what Garrus feels is outrage I think. He sort of looks around and goes "Don't you people SEE this mess? Why are we allowing it to continue?! It doesn't make sense! Why aren't we DOING something?" I think Batman and the Punisher want to clean up their neighborhoods and maybe face some demons from their pasts. I think Garrus wants to wipe evil from the face of the galaxy. He wants nothing less than to "undo" it. Obviously, that's impossible and I think he knows that and would say it. His head would say it anyway. His heart...
In short, Garrus is Garrus. I can't think of another character quite like him. His single-minded focus on his goal to the point that he rides himself into the ground on it reminds me a little of Roland Deschain from the Dark Tower or Woodrow Call from Lonesome Dove. Those guys don't act like Garrus, but they think like Garrus in some ways. They all have this set definition of how the world should be (an ordered, good place with justice, fairness, etc. etc.) and if the world doesn't match it, they want to make it match. They don't understand how it's even possible for the world not to match. It doesn't make sense to them.
I think King once described Roland as something "the deep romance of his nature was buried like a streak of fabulous alien metal in the granite of his practicality." Something like that.
Yikes, wall of text, and obviously this is just my take on Garrus.