In "Death Note," the theme of the cost of seeking vengeance is explored through the main character, Light Yagami, who initially believes that using the Death Note to kill criminals will make the world a better place. However, as he delves deeper into his quest for vengeance, he becomes consumed by power and loses his sense of morality. The series portrays the consequences of seeking vengeance, showing how it can ultimately lead to one's downfall and corrupt their soul.