Do motives matter?

Consider this statement:

"American leaders care about the Middle East only for its oil."

Now think about the possible contexts. Perhaps the speaker believes that American military action in this region is unjustified. But what do the motives of American leaders have to do with that? Either there are good reasons for taking such action, or there aren't. If there are good reasons, they either trump the prima facie reasons against such action, or they don't. The reasons bearing on whether an action should or shouldn't be performed are independent of the actor's motives.

John Stuart Mill put it best in Utilitarianism, chapter 2:

He who saves a fellow creature from drowning does what is morally right, whether his motive be duty, or the hope of being paid for his trouble: he who betrays the friend that trusts him, is guilty of a crime, even if his object be to serve another friend to whom he is under greater obligations.

That doesn't mean that motives are morally irrelevant, of course. One is likely to assess a person's moral character on the basis of his motives. When a person brings about good consequences as a result of greed, we are still likely to think less of him, even if we recognize his actions as justified. But we often don't have very good insight into people's motives. That sure doesn't stop many of us from ascribing them. We often infer motives from what we think of the person.

But isn't this circular? We study motives in order to assess character, but we come full circle when we infer motives based on character. In that sort of case, one is not proving anything about bad character by talking about motives. The person is already assuming bad character.