Law Abiding Citizen/WMG

Detective Garza is actually Sgt. Charles "Carlito" Grey.
Garza is a relatively minor character, but he always seems to be right about his guesses, is able to get to the scene of the climactic assembly scene in record time, and oh yeah, his specialty just HAPPENS to be Explosive Ordnance Disposal... just like the erstwhile sergeant. If you factor in that Clyde, he seems the perfect kind of person that Snake Doctor would want kept tabs on.

Clyde is an alternate version of the Riddler.

 * This troper wants to read that version of Batman.
 * It seemed like he was the Dark Knight Trilogy version of the Clock King

Clyde is an alternate universe version of Light Yagami
Brilliant mastermind arranges his own imprisonment as part of an overly circuitous plan for the sake of great JUSTICE!!! Check.

Clyde and moved to Gotham City where he became known as the Joker.
is where he got the scars. He is continuing with his goal to tear down the system but has also learned his lesson about how pathetic attempts to control things really are...

That really is Chief O'Brian there, thanks to time travel.
He was sent back to help stop Clyde and prevent a time paradox... Or something.

Nick's gun isn't loaded.
He just wanted to appear more threatening.

Clyde dying at the end was part of the plan.
All through the movie Clyde tries to hammer one point into Nick's mind, that the system is flawed and you have to take justice into your own hands. No matter what Nick does, he fails the entire movie, until he takes the law into his own hands, and murders Clyde at the end. That's what Clyde wanted the entire time! He succeeded, he took his revenge, he made an example of the justice system, he converted Nick to his own way of thinking, and above all else, he died, because he wanted to. That's the best part of the gambit. Clyde didn't get outwitted by Nick, he forced Nick to kill him.
 * Motion seconded by the fact that Clyde realizes he has nothing to live for anymore. No family, no real career, nothing at all. It's not even taking the law into his own hands, however, it's proving that the justice system is very flawed and corrupt. He wants people to know that they can't wait until more people die to deliver the correct kind of justice to the criminal who committed the crime.
 * He also successfully taught him not to make deals with criminals.