It's the parentheses messing upp your reading, sorry about that (it's one dead solar system population not one dead person).
Given the things that can increase you dark side score you would claim over 75% of the population never did any of this (of an unspecified solar system as well).
For insatnce: Nal hutta+ Nar shadda a non representative example, I would be quite surprised if the (playable living species) population of that solar system would not be mosty dead given both the direct and indirect damage caused.
note: this assumes that non-heroics "can't" atone (since they lack force Points) and that many/most heroics have >0 dark side score