By skyforbes This is why he wears the jacket with a scorpion and is obsessed with the fable about the scorpion and the frog.
It's a symbol to God of his self awareness and desire to reject his nature.
When Irene's car breaks down so conveniently, he thinks that God is telling him that his desire to just be human is supported. I do believe that he has had a good eye towards Irene for some time, and I do believe that he falls in love with her.
When he asks Benicio why he thinks the shark is the bad guy, he does so because he's fixated on the fact that humans tend to view brutality in general as evil or repulsive and he himself has a brutality that is tied to delivering divine justice so he feels monstrous.
He gets roped into the plot because he wants to save Irene and Benicio, but doesn't understand until the elevator scene that the actual purpose of God connecting him to Irene was to enforce his role by forcing his hand. Meaning he must use his powers again to protect her.
In the elevator, he puts a ward on her by becoming her guardian angel, and thus he must deliver divine judgement against those who's threatening her. He then sees that she is horrified by what he is, and it upsets him greatly.
He now has to go kill Nino, but he decides to put on a mask in defiance to God because he wants his face to still represent a human. He kills Nino by very deliberately reenacting the fable about the scorpion and the frog because he wants to tell God that he doesn't want to do this, God has forced him be the scorpion.
When he asks Bernie on the phone if he's heard the fable, he's actually telling him that he's going to kill him too if he comes after Irene. When he talks to Bernie in the restaurant, driver's look says that he knows Bernie's lying when he says the girl is safe. He smiles because he realizes that God delivered her to him because if he didn't get involved with her, she would have died.
In the end he understands that his nature was necessary to save her, and that it was not forced onto him. He did it because of love. The scorpion is good and it's necessary to protect the innocent. The light shining on him could indicate that he either makes peace with not being human and finds new purpose, or that he can still try to be human without rejecting his nature.
There's a lot more to this, including choice of lighting and such throughout the movie, and other hints here and there in various scenes, but that would be too much writing for me.
I'm pretty sure this is what the movie's about, or close to it at least.
Let me know if there's something you think refutes it, I might have forgotten some stuff!