I don't know if what he said was the truth or not, but I feel like, combined with the next chapter part, he did it so that he could ask her never to forgive him. He wanted her to cut all remaining ties with him so that she can no longer be burdened with being his friend. He believed that he had no right to further torment her.
Maybe he is being truthful and would have cursed her. But I want to believe that he wouldn't have. If he did, he wouldn't have tried to help stopped Luna.