Let's say you are a detective investigating a murder for hire. Why did the hitman kill this person? If you stop your questioning at "Because he was paid, duh"

Let's say you are a detective investigating a murder for hire. Why did the hitman kill this person? If you stop your questioning at "Because he was paid, duh" you're a really crappy detective. A good detective would then ask "Okay but why was he paid to kill this person?"