I received a zero on my essay because I disagreed with the prompt. It that ethical?

I completed an essay on Frederick Douglas's Autobiography. The prompt was "discuss how he uses literary devices to persuade the reader that slavery is evil." In a nutshell my thesis was, "He doesn't." I did outline the devices he used, but I showed that they were ineffective. I received a zero because I disagreed with the prompt. It was an attempt at playing devil's advocate. Should I have received a zero?