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A very simple answer, in my opinion, is "Trust in the Lord with all your heart and lean not on your own understanding" Proverbs 3:5.
There is no way to fully understand the Will of God, the love of our God, His power and His purpose when we try to use OUR understanding. His wisdom is so beyond our understanding, but His Will and purpose are perfect. So, allowing everything to happen the way He allows, may seem unreasonable, but then, are we trying to understand His purpose with our own wisdom of understanding or truly Trusting Him with all our heart?
Something else to think about... Without evil we would not know HIS goodness.
Any time something is "allowed," permission has been given. When I was growing up, I was "allowed" to do something after I asked and was given permission. That's the meaning of being allowed to do something.
God never gave permission for anyone to sin. Nobody is recorded as asking permission to sin.
I understand the gist of the question, however. All of these "why did God allow" questions are designed to lay at least part of the blame for how corrupt the world is on God. HE ALLOWED IT!! HE COULD HAVE STOPPED IT! Why doesn't He?
I don't think anyone who knows God through His Son Jesus thinks this way. Many believe there is a supreme being and at the same time, don't really know Him in a personal way. So they don't understand Him.
If God is omniscient, then He knows everything that will happen before it happens. If He's omnipotent, then He has the power to prevent the things that He doesn't want to happen from happening. So when things go down that are over-the-top sinful, like babies being thrown into the fire, or children being victims of child pornography, He either doesn't know about it, knows and doesn't care, or He knows and doesn't have the power to stop it. That cancels His omni credentials altogether.
Human arrogance is behind that kind of thinking.
Neither Satan, his cohorts, Adam and Eve, or anyone since them, has asked and received permission from God to sin. God doesn't "allow" sin. God forgives sinners who ask to be forgiven. He "allows" forgiveness..