I think if there's a God it cannot do logically impossible things, and logic may necessitate that anything that can exist exists, including unjust worlds. Also it may be that just as a particular sperm and egg needed to come together to make a particular individual be born, it may also be that a particular life history may be necessary for that individual to exist as well. Clearly without their life experiences, in addition to their genetics, an individual wouldn't be who they are.
Just as there are people born into uber wealth, some also having uber good health, while others may be born into uber poverty and uber bad health, even greater extremes in differences may exist. There may be individuals who exist in paradisiacal worlds and others that live in hellish worlds. Some may even be immortal within these worlds, forever experiencing wonders or tortures.
Heck it may even be that for every good feeling experienced somewhere it balances out with a negative feeling being felt elsewhere, thus balancing net feelings into zero.