Genocide.How murder hobo do you have to go? (scale of kill every mob to just the minimum)
Mobs don't respawn, there are story checks based on your protagonist's level so you are encouraged to be as high level as possible, some optional fights are fairly difficult.
Only in some specific events the game hints that you shouldn't kill some specific mobs, but otherwise you are encouraged to kill everything you can.
The game is beatable even if you just kill the bare minimum but it won't be a good idea if you want a more "completionist" playthrough.