Switch(2008)

Switch

Two newbies at the Narcotic Control Department in Japan, Haru and Kai, struggle against: drug dealers, gangs, murderers, psychopaths, mysterious organizations, and mad people in general. The gentle and caring one of the group, Kai, who has problems of his own, has the tendency of turning into a lethal killing machine in certain scenarios. The way he acts changes like that of a "switch". As the two do their job, answers of what is behind Kai's strange behavior and who exactly he is, is revealed.

(Source: Anime News Network)