What is mortality rate in dragon ball and why they considered destroying universes for that? I'm not able to understand what is the meaning of mortality rate as referred in episode 84 when it is said that universe 7 has low mortality rate so what they meant with that and why they considered destroying universes for that.
According to Dragon Ball Wikia - Mortal Level


  Mortal Level (人間 レベル, Ningen Reberu, lit. "mortal level") is the score
  that the Zeno and Future Zeno use to rank the twelve universes. It is
  a measuring of the average development, or "quality", of the mortals
  existing in a universe. The Mortal Level of a universe is directly
  related to the effectiveness of the jobs of its Supreme Kais and Gods
  of Destruction (allowing and promoting the creation and development of
  adequate civilizations and elimination of inadequate ones).