Gerard Butler was originally slated to play Jamie Foxx's character, Nick Rice. Presumably they just couldn't figure out an excuse for the Deputy DA to gratuitously take all his clothes off, and so switched him over to Clyde Shelton, a man at war with 'the system'.
Part of the problem with the plot here is that audiences can be forgiven for being unsure which of the two main protagonists they are supposed to be rooting for. In the end the balance just about favours Rice, but perhaps only because Shelton turns out not to be a 'law abiding citizen' after all, but some sort of shady government assassin. One therefore has to speculate why he didn't just deal with his problem by terminating the murderers of his wife and child before the suits at the DA's office had had a chance to cut a deal with one of them.
This is a medium-high concept thriller about a man in jail for killing the scummers that killed his family, who somehow manages to extend his revenge to all the Justice Department officials complicit with the original plea-bargain (...though bizarrely stops short of whacking the guy who actually set up the deal).
Of course it's all very silly, but mostly the entertaining kind of silly, and I don't really mind it when the constraints on believability are part of the essence of the movie's proposition and neither detract from the tension nor insult one's intelligence too much.