Unfortunately, the same thoughts and actions can correspond to different shades of good and evil, even in the mind of the same person [emphasis mine]

This shows that there is disagreement & confusion about what is good & what is evil. That no more proves good & evil are meaningless, than disagreement about physics shows that physics is meaningless.

Actually, disagreement tends to support the opposite conclusion. If I say fox-hunting is good and you say it's evil, although we disagree on fox-hunting, we seem to agree that only one of us can possibly be right. At the very least, we agree that only one of us can win.