>>281154888 (OP)a lot of people seem to not get or not accept the logic of Johan's motivations and just take at face value what the neo-nazi's said
Johan thinks forcing Dr Tenma to kill him in cold blood will make him just like him specifically because he mistook Tenma's choosing to safe the life of a monster child over the life of a wealthy man as an act of inhuman kindness, monstrous kindness.
So within his psychosis, that made Tenma the other half of the Nameless Monster, the one that learned to live among humans, and that their story had to end in Dr Tenma being 'devoured', leaving only a single, complete monster. and Johan needed this to be true because he needed to be the main character of The Nameless Monster as proof he wasn't just a spare copy of his sister.
The Neo Nazi's didn't even realize Johan had never been to the Book House. If their crackpot theories had worked, it would have been Johan's sister that would become the next Hitler, nor would Johan have been able to wipe them out instead of obey them