 If you liked
Devilman
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 ...then you might like
Monogatari Series: First Season
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(As multiple arcs are included in Monogatari Series: Fist Season entry, this reccomendation is mainly for the Kizumonogatari arc.)
The protagonist is transformed into a half human-half monster (vampire/demon) thanks to an interraction with pure monster - regular vampire and regular demon, though in both cases the the monster is one of the most or even the most powerful monster of their kind. This transformation is done against his will forced by the circumstances and because they wanted to help someone. Both stories involve plot twists.