If Quirrell is lying, then asking the question "Why not use the green stunner in other circumstances where Avadra Kevadra was used?" may lead to that lie being uncovered.

I admit I had not considered Inferius on the centaur. However, I rather suspect that Quirrell is priming Harry here; he does something (hitting a centaur with a green stunner) that looks evil, then demonstrates that it is less evil than it was. In the future, then, Harry will be more inclined to believe that Quirrell has done something less evil than it looks like he has done; he could, for example, use Avadra Kevadra on Dumbledore later (making it look like a green stunner to Harry) in circumstances where a green stunner would not be evil and then rely on Harry to prevent any immediate revenge against himself (long enough to portkey away at least).