Jesus died for everyone. “For God so loved the world that He gave His only begotten Son,….” John 3:16 (NKJV). Hitler and every human being were redeemed when Jesus died on the cross. “As in Adam all die, even so in Christ shall all be made alive (in the 1,000 year Resurrection Day).” 1 Corinthians 15:22.
God’s love for the world in John 3:16 is the Greek word “agapa?”. It means God will only do what is good for each person. God’s agapao love is not a tender feeling. It is the will to do good.
In the resurrection, not everyone will learn to love God and righteousness. Those incorrigible (which may include Hitler) will die the second death and never exist anywhere again. Ezekiel 18:20 (NKJV), “The soul who sins (in the resurrection) shall die. The son shall not bear the guilt of the father, nor the father bear the guilt of the son. The righteousness of the righteous shall be upon himself, and the wickedness of the wicked shall be upon himself.”
If a person doesn't learn to love God, then that person would be out of place and miserable in God’s eternal, righteous kingdom. So out of agapao love, God will end that person’s existence. Ending that person’s life will be a demonstration of merciful love.