 If you liked
Monster
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 ...then you might like
Yakusoku no Neverland
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Main villain (Johan) comes from an orphanage where kids were experimented on by Nazis and is equally (if not more) terrifying as Isabella / Mama giving similar tensity whenever he is on the screen.
Both shows explore depths of human mind, making you question what is right and wrong, as well as an Mystery element having you grave for more with ending of each episode.