Righteous Kill (2008)

Turk and Rooster, two aging NYPD detectives and longtime partners, are hunting a serial killer who is murdering sociopathic criminals. They both have personal issues, and when they start working with a younger investigative team, Perez and Riley, tensions between the two teams is inevitable, especially since Turk is now living with Perez's ex-girlfriend, also a homicide detective.