As with many vigilante movies, Law Abiding Citizen posits the argument that there are flaws in the justice system. Clyde isn't only interested in punishing the killers who ruined his life; he wants to destroy those who enabled them to walk free in the first place. Clyde's final mission sees him trying to blow up City Hall after the mayor calls an emergency meeting, with all of the judicial figures, while they're all in one place, but Nick is around to thwart his plans. While Clyde's plan ultimately comes back to bite him in the end, it's clear that he was motivated beyond simply killing some politicians and criminals. He wanted Nick to learn a simple lesson: don't make deals with murderers. Essentially, Clyde became a ruthless killer just so that he could change the attorney's worldview, even though that meant him becoming a sacrificial lamb. Then again, Clyde's only purpose for living after his family's death was causing carnage.
On the flip side of the coin, Nick's dealings with Clyde have clearly changed his own outlook. By placing a bomb in his cell, he shows that he's also willing to go to extreme lengths in order to achieve his vision of true justice. Until this point, Law Abiding Citizen is concerned with exploring the ambiguities of the system. However, the ending proposes that capital punishment is the only answer for certain criminals.
