I’m investigating Christian theology.
Hitler is just an eye catching hypothetical. The most extreme example of an evil human being one can think of.
Let’s talk about a real world example of this hypothetical - Jeffery Dahmer. Jeffery Dahmer became a reformed Christian in prison. He was baptized and had weekly Bible study sessions. Is he in hell or heaven? What does Christian philosophy say?
Obviously only God knows the heart but according to scripture, obviously we won't know until we go to Heaven, the answer is that "Yes, if he had an encounter with Jesus--he has the potential of being one with God."