Siva no Yoru

By day, Shiba Takumi is the head psychotherapist of a police headquarters infirmary. By night, he kidnaps people and messes with their minds in the name of justice and experimentation. He selects his subjects carefully to be people who make the world around them a worse place. Then he drugs them and whisks them away to his laboratory, where he subjects them to an experimental treatment that changes their personalities. It doesn't always work exactly as he intends, but there's always tomorrow to try again...

(Source: MangaHelpers)