Maybe it's because he honestly believed himself not to be and thought that it's good. You know - a good person in the constraints of his time. After all, even in this poem he postulates that an Odious Woman may mend - much more than many of his contemporaries would do. And he was so careful to acknowledge the basic humanity of the savages, to note and praise their nobler qualities, and so proud and happy about Britain's complete victory over anti-Semitism.