The logic of "allowing pain & suffering for a good reason" requires one to treat people as some sort of collective blob where individuals don't matter. A good father does not allow his children to destroy themselves or to kill & rape & enslave each other. Those behaviors would be indications of a father's failure.
Discussion
I don’t follow your blob comment.
The dentist analogy is just stating that a child with limited knowledge may think his father hates him for taking him to such a dreadful place. We are such small children. It’s perfectly logical to say that this argument is not necessarily valid because we don’t have all the counterfactuals as fallible humans. In other words, we think the dentist sucks, wait until all of our teeth rot out. The father cannot avoid the dentist. God cannot force humans to be perfect.
Furthermore, killing, rape, and enslavement are all just words for extreme sin. If God could have a good reason for allowing me to hit my finger with a hammer, I don’t know why he couldn’t use extreme evil and abuse for good also.
In fact I know many terrible stories where people, while not justifying their abusers, see that they had valuable lessons and could help others in the midst of their pain.
There's no way to help the ants in an ant farm without wrecking the whole thing