In the end of Law Abiding Citizen, Clyde Shelton was being blown up by his own bomb but in the end, even though he died, did he get what he had always wanted? When he was being blown up by his own bomb, there are scenes that focus on his facial expression while he was holding the gift his daughter gave him. What does that facial expression mean? Somebody said it is relief, somebody said it is guilt, I'm just not sure.
Burhan already makes a great point in that Clyde was merely exhausted from his anger and had nothing to live for anymore, so he was partly relieved to finally die and get back to his family, something he himself couldn't and wouldn't do. And I also agree that it surely isn't really guilt over what he did.
However, there is yet another aspect to this. The primary motivation why Clyde did this was not only revenge, but also to prove his point. He wanted to drive the law enforcement system to its extremes, so that they finally realize what is "necessary" (from his point of view) to stop somebody and enforce true "justice", and not hide behind their supposedly broken system anymore, which was what he primarily blamed for the lack of justice the murderers of his family got.
Throughout the movie he tried to show everyone how broken the system is, he made deals for trivial commodities and baited a judge into giving him low bail, only to then insult her for her lack of justice, and ultimately comitted all his crimes being locked up in prison. He forced the police, the attorneys, and the entire city's government into a corner from which they couldn't get out. There simply seemed to be no way to stop this guy with legal measures.
So when Nick gives Clyde a last chance to give up before actually deliberately blowing him up, he has crossed this line. Nick effectively executes Clyde for what he's done, without a trial. Nick is playing judge, jury and executioner in unison. So in a way Clyde ultimately has won as this is what he wanted to achieve all the time, Nick has finally "understood".
In the end, that was what he wanted - to be with his daughter and wife.
The rage of the way his wife and daughter were killed and the deal that was made drove him into this madness for revenge which was all consuming.
He wanted relief from that, which is what he got in the end when he was finally trapped by his own bomb.
It most definitely isn't guilt; as he never shows this expression throughout the entire movie.
During the movie we can see something like shame in face of Nick, because he himself knows that he has made a big mistake, especially in relation with murder of Darbi. He could compensate his mistake with giving a chance to Clyde, because his big mistake had led to a big illegal abuse of the law. On the other hand, he is a greedy one. If I were Nick I let him go, after that I tried to solve the problem in other ways. Clyde is a genius, it is too important. There are lots of things about this movie...