Every system cracks under a mistake from time to time. Even the one of the Shinigami, whose legend should be known to everyone. When a human commits suicide, their soul gets bound to the earth as punishment and they have to keep on living as a Shinigami and collect the souls of the dying. Like that they are forced day after day to watch people find the peace they longed for so dearly. Hence why it is a cruel fate to be made to live this existence despite just being an unlucky soul.