Rules of Engagement.
Let me explain.
God’s Laws of the universe He has laid down from before Time. He is 100% Holy/Good. Evil (opposite of Good) exists.
When he created Man to have a relationship with, the Bible said God created Man in His image - i.e., ability to love, have free will. God had that beautiful relationship with Adam and Eve - but said ‘Do not eat of the Tree of Knowledge of Good and Evil’. You I’m sure know the story well. Satan (fallen angel) twisted the truth (Jesus said Satan is the father of Lies - i.e., his M.O. is to twist Truth). Eve was taken by what Jesus called the Lust of the Eyes, Lust of the Flesh, Pride of Life (’you shall be as Gods’ - back to what I said we are tempted and sin being mini-Gods).
The Bible states that therefore the entire creation suffers from Adam’s disobedience. Curse of Sin.
Back to God’s Laws. There is penalty for Sin. Sin (disobedience to God) will ALWAYS lead to harm of self or others (human or nature) - it’s what Sin is - it would not BE sin if it did not lead to harm or death.
But, then, God cannot change His Laws. He is true to Himself. While HIS solution (as planned from before the foundation of the World was laid down) - i.e., His Son’s coming, dying and breaking power of Sin and Death through his resurrection) was being implemented, we humans through the course of time, live our lives. We have individual choice to follow God’s ways - and he gives EVERYTHING necessary in FAITH *IN* Him, to have forgiveness AND that He rewards us with eternal Life for ACCEPTING His substitute payment.
BUT - Hitler and all other evil men CHOSE their fate. BUT the cause/effect of their choices STILL emanate throughout the earth - i.e., again God’s Law of the nature of Right/Wrong, Good/Evil, Life and Death, Sin/Righteousness - cannot be changed.
THEREFORE - God’s plan of salvation and redemption for each individual does not negate the effect of OTHER individuals’ evil choices already made - but those who God CAN show His Love and glory and holiness and goodness and kindness and power - then THEY can be used to manifest to the remaining OTHER living humans the POWER of God’s goodness and grace. They also - as Jesus said - be Light and Salt (i.e., vessels of God manifesting HIS goodness) to those around them.
Short answer - It’s the nature (construction) of things. Things play out according to the laws of the universe God has created. Good and bad. Because, at the end of the Day, the only way God honors Himself (His Name, i.e., integrity of who he is) - is by His Laws taking their course as designed but also then His Love providing healing of the ill effects of Evil.
Am a bit reluctant to continue this thread. However still not satisfied that you answered the question just why God with fore knowledge of all things would created a creature like Hitler. Consider a devout bewildered man who arrived in a concentration camp, had his wife and children forcibly taken from him and that night hungry and cold in a barracks surrounded by fifty other such men, prays fervently for help. Yet none comes.
Not the first time such questions have been asked. Christ himself did not call on legions of angels to save himself and the good thief from the horrors of the cross. Is the solace that He, the good thief , the man in the concentration camp and his family are now, despite their worldly suffering and injustices, forever in paradise and Hitler is with the cruel unsharing wealthy man in the parable eternally damned? Why would God create such imperfect creatures who’s fate is to be eternally damned?