The Duke’s ego probably made him an easy target for the Red Death because he felt that he was invincible due to his prominence and wealth. The Red Death plague was spreading across the countryside, and the Duke decided to gather up 1000 of the most noble in his kingdom and sequester them behind locked and bolted doors in the abbey of his castle.
Much to the Duke’s surprise, the Red Death was able to sneak into his castle and kill the Duke and his visitors. Presented as a man in form, the Red Death did not discriminate. The plague didn’t care who contracted it, and no amount of money would keep the Duke from being its victim. So while the Duke’s fellow countrymen were dying, the Duke felt he was above them and couldn’t be touched. It was his ego and attitude about his own mortality that led to his death.