Ragabul the Ontarah wrote...
I thought that Alistair was a bit more idealistic than Garrus. He wanted to believe that the Grey Wardens were paragons of virtue. Heroes. That their actions were noble. Duncan was the face of the Grey Wardens for Alistair - the one who saved him from the drudgery of the Templars, and had given Alistair another chance at life. So when the Warden spares that certain character, Alistair is absolutely shocked because the Grey Wardens, to him, were supposed to be
good people.
So to relate that to Garrus, Garrus does not really have that same complex, or to the same degree. It would take a
lot for Garrus to leave, so much so that it's hard to imagine him leaving Shepard in a believable way. The most plausible scenario, I think, is that Garrus would be horrified at something that Shepard may do, and try to act against that. Not act against
Shepard, but the action itself. Try to stop it from happening. It's a fantastic parallel to what could have happened with Sidonis - only this time, Garrus is blocking the bullet from the target.
If Shepard does go along with this action that shakes Garrus, perhaps when you speak to him on the Normandy again, he says "I'm sorry Shepard. I have a lot of things on my mind. I need to think about what happened" - in other words, he won't speak to you because he's so caught up in what just happened.