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).