I remember that scene. What struck me about it was how it was obviously not the character's first choice to kill the rat. He spent quite a bit of time trying to shoo it away or discourage it, and only stabbed it once the squeaking got so loud that would reveal his position.
While I feel sorry for the imaginary rat, he was a bit of a dick. Oh, and I also feel sorry for the 500 Russians and Arabs I killed over the course of the game. I guess.