Hitler claimed to be a devout Christian ........that did not make him a Christian. Men have free will.......Hitler and Hitler alone made the decisions/judgments that contradicted the will of God, no one else. Examples of men's free will, (Deut. 30:19, Josh 24:15, Gal. 5:13, Gen.2:16-17, John 7:17........)
Example, God allowed Pilate to hang His only Begotten Son on a cross........ why? Because no one knows the plans of God except those things/subjects He decided to reveal through inspiration of the Holy Spirit (there are some 55 passages that explain this in detail with no ambiguity)......No one knows the mind of God, not even Jesus Christ knew all things. (1 Cor. 2:11, Matt. 24:36)
But of course, YOU KNOW more than what is written/revealed. "Do not go beyond what is written......." -- 1 Cor. 4:6
I do not claim to know why God allowed Hitler to rise to power........but we do know what was revealed, Hitler and His Socialist empire was brought to kneel before the combined strength of a few righteous world leaders.
What? Are you attempting to declare that God allowed Hitler to win? No........of course not, God did what God does. When Hitler demonstrated that he (Hitler) was not a righteous agent in protecting and defending those under his authority in Germany and elsewhere ......... "............But IF YOU DO WRONG..........BE AFRAID, for he does not bear the SWORD (capital punishment) in vain." -- Romans 13:4
In the previous passages, God explained that Vengeance belongs to God Himself. "Do not avenge yourself.....leave room for God's WRATH." -- Romans 12:19