I believe in the providence of God...that nothing happens that He does not permit.
Then why did God let this happen? Why didn't He stop him from doing what he did?
I don't know the answer.
God allows all humans and animals free will. God can't break the free will of anyone. Only God (all three), angels, Satan, demons and fallen angels have no free will. Those do good or evil and they've already chosen their path totally. None of those can change. Only we can chose good or evil.
God didn't let anything happen. He allows free will and people choose to do good or evil or neutral things.
Think of this way. If God controlled everything, we'd all be robots with no independent thoughts and actions.
What doesn't God stop evil? Free will. He can't. Now, he may get pissed there is lots of bad at someone and decide their time is up, but that's pretty much it.
People make choices. Good or bad or neutral.