This bothered me for a while, but I figured out something that satisfies me and makes sense for the Imperials.
Dark Side means following your emotions. NOT NECESSARILY "bad ones"! Love or compassion lead to the Dark Side too. On the other hand, Light Side means peace, calm and rationality.
If you shoot a prisoner, either because you want to enjoy the act (hatred) or because you want to spare the prisoner pain (compassion), well, you are acting on feelings - that's DS.
Usually, interrogation would serve the Empire better. It's a feeling-free, rational decision to make. Hence, it's LS.