An external reason claim is one that is applied to the subject of the ascription regardless of what are his desires; and Williams argues that although we make such claims, they are all false. (p. 108)They are false because reasons for action must be able to explain the action in question, and to do this the alleged reason must be able to motivate the agent. Joyce accepts the Humean view (without claiming to be able to prove it correct) that actions are motivated by beliefs and desires working together.
There is clearly a difference between the reason why someone did what they did and the reason why they should have done something else. I mean, there is a difference in the sense that these do not have to be the same thing. Perhaps you did something for the money but could have made more money doing something else, in which case both reasons are "for the money." But if you kill someone for the money then the reason why you should have refused is rather different. And it might not match any of your desires at all. It is still a reason why you should not have done it.
Is it a reason that you had not to kill the person? Maybe not. But it was a reason that, so to speak, existed for you not to kill them all the same. What kind of existence do reasons that are not acted on have? How can there be a reason that is not owned by someone, that is not someone's reason? And what is the reason not to kill people for money (if, say, Qaddafi makes you a generous offer)? There isn't much of a reason, in a sense. That is, there isn't much I can think of to say to anyone who seriously asked such a question. Because they are people, is about as good as I can do. But it sounds crazy, or like a joke, to say that there is no reason for people not to slaughter innocent civilians for money if that is what they want to do.