Hi,
I'm looking for a movie in which the policeman becomes the killer. After long time of seeking police finds a killer responsible for many victims but he commits a suicide(?). Anyway, The FBI (?) wants to know his informants and contacts so they decides that they will teach some guy to be him and live his life to get information only he could get. Only few agents know of this mission. Our policeman needs to leave his wife and kid to fully sacrifice for the mission. They are teaching him how to become this one special killer. He eats the soup that killer used to eat in his childhood, even smells the same smells just to know how he felt. He's becoming very intelligent person, he knows the person by looking into their trash, and just by seeing how the person looks.

In final action he's getting killed by police group who is not aware that he's actually a policeman.

What's the title? Please help:)