The film that began the McConnaissance in earnest, Killer Joe is a down-and-dirty thriller that sees McConaughey play a corrupt sheriff who offs people for fun/money.
Emile Hirsch is a washed-up drug dealer with mounting debts who decides to murder his own mother for the insurance money. Enter McConaughey who forms an unusual relationship with Hirsch and his character's sister, played by Juno Temple. Often disturbing and pitch-black funny in places and a fantastic lead performance by McConaughey, Killer Joe is recommended viewing. You'll never look at fried chicken the same way again.