 If you liked
Phantom: Requiem for the Phantom
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 ...then you might like
Grisaia no Meikyuu: Caprice no Mayu 0
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They both have male protagonist characters who show an exceptional ability to kill people. Grisaia's protagonist is more inhuman from a cruel, abusive, abnormal past. Both shows show the protagonists training how to kill people in both guns and knives. They both have their protagonists put into morally ambiguous circumstances, with a stronger case being able to be made that they are, the bad guys (for at least part of the series).
Main character is kidnapped by evil organization and trained as assassin. He is forced to kill against his will. Finally he rebel and fight against organization which destroyed his life. Main character is controlled by darkness, grief, lost meaning in life. He meets something special which helps change, find light in the dark. Both are great action, philological anime.