I guess there is a slight ambiguity in the way Nicholas Humphrey uses the word 'right' in the sentence: "none of this would give you a right to administer the poison". I doubt he is making a moral statement. What he is pointing out is that your beliefs will have to be judged by reality. Your beliefs do not affect the fact that what you are administering is poison.
In fact, he points out that having incorrect beliefs might make you morally less culpable. But it doesn't make you right.
Having incorrect beliefs and acting on them is the right thing to do. Acting on the right thing to do makes you right. I disagree with your reading of Humphrey's statement because the idea that you can be less morally culpable given certain conditions still seems to imply a certain degree of culpability.
Your beliefs have to be judged by your other beliefs because pure objectivity is epistemically inaccessible. The quote is at best useless because no one intentionally poisons their child if they love their child. The quote serves to make other people (eg us...
Here's the new thread for posting quotes, with the usual rules: