Or, rather, in a world where human beings are indeed beings endowed with free will, the possibility for evil and pain would be inherent to whatever system enabled human beings to at once be in the best place possible originally, and then have so much pain and sorrow in the end.
If human beings are to be free willed, for them to choose communion with God willingly, there must also be the possibility for evil and the rebellion against God for that choice to be anything worthwhile.
There are horrible things in this world, and this is because of the choice inherent in the system: t follow or disobey God, which are both as easily made.
The real question is where the notion that this world is in fact not the best of all possible outcomes comes from. If you're merely a byproduct of a cosmic accident, your thought process wold have no reason to enable you to think that you are in anything but the best of all possible worlds, for that would imply thinking in terms beyond the system itself.
For you to have a concept of "better" is very intriguing, and where that idea comes from is very interesting in and of itself. The Bible would say that we remember a time when we had communion with God, or that we long for it, without necessarily that being an active cognitive process... A God shaped hole.