Clyde spent 10 years planning every detail of his rampage, taking into account everything from obscure legal precedents to the probable funeral arrangements of Nick's understudy. The only thing he missed? Mere days before the plot, after he stopped researching and plotting, the understudy ditches her old man and gets a new one, Who has the ability to blow the cover provided by his Panama corporation.
More Fridge Brilliance: Clyde said that he was going to "bring down the temple" and that it will be "biblical". He hinted of his own death, just like Samson, which was part of his plan.
A Blink And You'll Miss It - When Clyde is talking to Nick outside the prison, he says "This is Von Clausewitz shit— total fucking war". This is in reference to Von Clausewitz's "Third Reciprocal Action", which basically says that to defeat an enemy you must completely annihilate them. That is why he wants to kill everyone in the Justice Department, and explains his targeting of the city hall meeting at the end.
Clyde offering his cellmate half of his expensive lunch. An act of friendship? No, he was giving him a last meal before his execution.
Nick insists that the system works, even though he literally let a violent criminal go, just so he could keep a track record going. Did he buy his way through law school or something? Compounding this is that letting a child murderer plea out would destroy any career he might have had as a DA.
Nick is an idiotic slimeball. Bribing his way through law school would not be out of character for him.
Clyde brings this up at more than one point in the movie; it finally sticks near the end. The fact that Nick didn't care or even tried to prosecute Darby is what ultimately kicks off the plot. The point of it all was to make Nick care about more than just his own career. The Justice System only works if the people in it not only follow the letter, but the spirit of the law. Nick didn't.