To avoid going into Outer Darkness for Eternity every person that ever lived will be Baptized at some point.
If the person does not accept the baptism in the next life, then they will go to Outer Darkness for eternity.
Based on Hitler's life, it is likely that He will rebel against his baptism, as would others like him will most likely do as well, and will not accept the Baptism of repentance, in the next life in the World of Spirits.
How many will reject the Baptism that is done for them is not known. But certainly some do. Generally speaking it stands to reason that the more wicked a person is the more likely that person will reject the Baptism that was done for them.
By doing this, they reject the last chance to avoid going to Outer Darkness for all eternal time. The best that such people as Hitler could hope for is to go to the lowest part of the lowest Heaven, if he were to accept his baptism.
This illustrates the massive difference between the highest Heaven, called the Celestial Heaven, and the lowest Heaven, which is called the Telestial Heaven.
Only those who repent in this life, and live the commandments of Jesus Christ zealously, will go to the Celestial Heaven. Whereas all people who murder and commit adultery and rob and participate in Secret Combination including organized crime, will at the very most, go to the lowest Heaven, after having spent time in Spirit Prison, after this mortal life is ended for them.
Therefor repent, and live the commandments of Jesus Christ, and join Jesus Christ's holy church, THE Church Of JESUS CHRIST Of Latter Day Saints, and continue to live the commandments of Jesus Christ more and more, after you have become a member yourself, and by so doing be an example of perseverance to other Latter Day Saints.
If you do not do this, you will not be allowed to go to the Celestial Heaven, anymore than Hitler will ever be allowed to go to the Celestial Heaven.
However, you might be able to go to the Terestial Heaven by God, if you are not extremely evil. That is the Heaven for those who are lukewarm, neither super bad, nor super good, but rather some where in between.
God bless.