In Man on Fire Denzel Washington plays a washed-out special ops agent named John Creasy who takes on a simple bodyguard gig in Mexico City for the sake of staying employed. The simplicity of this new job is fitting for him because he is still highly skilled in the killing business but his chronic drinking hinders his abilities. His assignment is to guard a little girl named Pita (Dakota Fanning).
The most dominant theme in this movie is justice, in particular God's justice. Before he gets "on fire" he speaks briefly with a Catholic nun who chides him about his job, saying she wishes it didn't have to exist. She seems to frown on him personally. She begins to quote a Bible verse, saying "Do not be over come with evil..." but Creasy quickly cuts her off, finishing the verse with "... but overcome evil with good," saying the verse number as well: "Romans 12:21," all this in Spanish. So yes, he is aware of the Scriptures. Yes he is aware of evil and good. Then he says to the nun, "I'm the sheep that got away."
I appreciate Creasy's desire for genuine spirituality. He's not going to hide the fact that he despises his existence. He's not going to deny that he has regrets. If God reconciles him, so be it. This helps him develop a pure sense of justice, which God Himself seems to instill in Creasy through his relationship with Pita.
At first Creasy is cold and distant from Pita. But he eventually warms up to her persistent amiability. Time goes on and, thanks to her, he learns once again how to appreciate genuine, untainted love. Then someone kidnaps Pita. The contrast between good and evil is so clear cut for Creasy at this point that he seeks justice for the girl, even if it costs him his life.
Man on Fire takes place in Mexico City where public officials and law enforcement are famous for their corruptibility. This means, if Creasy does not save Pita, then no one will. In fact, if Creasy wants to save Pita, he must do so despite Mexican law enforcement and public officials.
I can't say conclusively that Creasy is justified in the actions he took, but I can't blame him either. He saw purity in contrast to a world full of corruption. He saw a vision of justice more clearly than everyone around him. This is similar to God's point of view (not to say Creasy should have seen himself as God): God's hates injustice. He kills wrongdoers. He destroys whole nations that get too far out of hand. God especially hates the oppression of the helpless and the innocent people. So it is possible that God would use a man like Creasy to bring some balance to an excessively unjust situation.
Notice the heart with which Creasy rained fire on the kidnapping syndicate. His actions brought him no personal gain. He even ended up giving his life in exchange for the girl's. This is exactly the way Jesus Christ works. Christ will bring justice to the world in the end times, but not before He died to save the world.