Repentance is one of the seven things God created before He created the world. It has a supernatural power to change a person.
בס''ד
There are sins in Judaism that are unforgivable even after repentance. I brought some obvious examples earlier in this thread which you are ignoring.
Another prominent example is King David, the most righteous king in history. The Mashiach (Messiah) will come from the lineage of David. David repented intensely after the sin of Bat Sheva and Uriyah the Hittite. David tore his clothes, sat on the floor and fasted 7 days. David begged Hashem to forgive him and begged Hashem to spare the life of the baby boy that was conceived as a result of his relations with Bat Sheva. But David's action caused the death of Uriyah and Hashem refused to forgive. David's baby boy died. And that was just the beginning of the punishment. Because of this terrible sin, David was unable to give a moral example to his children. David's son Amnon raped Tamar. David's other son Avshalom rebelled against him, removing David from the throne and causing a civil war in which 20,000 Jews were killed. David's son Avshalom was also killed in the battle. None of these terrible things happened until the Bat Sheva-Uriyah sin. Before the sin, everything David did was incredibly successful. Even at the end of his life, David paid a tremendous price. David became ill in his sixties and died at the age of 70, even though a righteous king like him should have lived a very long and healthy life.
I could give many other examples of how Hashem will not forgive a sin like intentional murder without punishing the murderer.
Once again, if what you are suggesting is correct, then Hitler could repent and be forgiven for murdering 6 million Jews (G-d forbid!). A god that would do that would be a monster.