Law abiding citizen

This is my favourite film. It's a revenge story. 
During a home invasion, Clyde Shelton witnesses his wife and young daughter's murder at the hands of Clarence Darby and his accomplice Rupert Ames. Prosecutor Nick Rice claims that the case against Darby is weak since the DNA evidence was ruled inadmissible, even Clyde's testimony alone is insufficient to prove either suspect's guilt. A deal is made whereby Darby will testify; sending Ames to death row in exchange for a lesser sentence. Since Darby was the actual killer, Clyde is left feeling betrayed by the justice system.