Now that his brother is gone he had all the time in the world to research how to undo the sleep spell (as well as having access to most powerful magician ever - True Iremi), yet he choose the easy way out of dying and hurting his loved ones in the process. And he did not even consider the possibility of his brother simply lying to him out of spite, and him killing himself would not undo the sleep curse - it would have been extremely funny if author have gone for that ending, where Ethermask kill himself and nothing is resolved.