Assume god exists.
If a just god exists, how is it that evil exists in the world. A better question. How is it that newborn babies suffer, that people are slaughtered by others stronger than them. Why the pain? Let’s call the existence of suffering, especially extreme and unwanted suffering, the problem of suffering. Is there a solution?
One solution is that some people deserve to suffer, but it often seems that good people suffer too and suffering is not in proportion to guilt or wrongdoing. Does a newborn baby choking to death deserve to die? The answer seems to be no.
Another explanation is that god cannot override free will, and so must allow suffering to happen. The problem with that answer not all suffering is a product of human choices. If a meteor strikes the earth in 3000BC and wipes out all life, is that really the fault of the people alive at the time.
Another explanation from the author of Slate Star Codex is that god creates massive numbers of parallel worlds. In each world there are slightly different versions of us. The me who suffered is a somewhat different person from the me who did not suffer. To allow all possible people to live, all lives which have value, god creates worlds which are perfect but also worlds where those lives/minds which are born from suffering exist.
The final explanation is simpler. There is no suffering. The moment a person is about to be tortured or to die or to suffer, they are lifted from the world and into a simulation. The simulation may be the afterlife or may be a near identical copy of their world. In their place remains their body and a simple non-sentient program which mirrors the responses they would have given, the ways they would have acted. There is no problem of suffering if there is no suffering.
Another variant. Maybe it’s all an illusion. Maybe we never suffer. Maybe any memory of suffering is fake, implanted by god. Maybe we live a life free from suffering, only remembering fake suffering enough to shape us into the minds we need to be but never experiencing it and hence never being harmed.
Finally, the pessimistic explanation. Maybe the problem of suffering isn’t a problem. Maybe we all deserve to suffer. Maybe our suffering is good, just. Most of us for most of history where violent, hateful, rapists, murders, cowards, monsters. If we’re not today, that’s because of our environment, not because we are innately are good. We all deserve to suffer. Even a baby deserves to suffer. It’s just as evil, just as optimized by evolution to maximize it’s genes chance of success and little else.