I'll elaborate more. Get ready.
The movie isn't that hard to get. Really. Anyone with a brain can see what this movie was trying to say. It's not even something hidden below the surface. It's right there.
Judge Dredd is not the main character. It's not his story. It's Anderson's. Dredd is just the force behind her story. Judge Dredd feels one way about the law. Good and Bad. Black and White. Zero middle ground. You do the crime, you do the time. End of story. We see Anderson as a Judge who feels indifferent about these "laws" and the way the Judges dispatch them. When she sees the man who tried to kill her, she's got her gun pointed at his face, she feels sorry for him. She knows, and understands what he's dealing with, because a few years ago, she was just like him.
But...she also knows what he did and she is trying to do her job, and she has to do what she is told. Bam. Judges don't care about the consequences of their actions. They don't care who this man was. Justice is served. The end. And when Anderson finds out the man was a husband, probably loving one too, with a wife who clearly feared for his life, she feels the pain caused by the Judges. And that messes her up.
Until she finally meets someone who truly IS terrible. That's where she fully comes into her own. She realizes that the Judges have a clear purpose, and it's to deal with people like Kay, and Mama. People who are beyond saving. But that doesn't mean she sees things in black and white. She spares the Eye Guy because he had no choice, and he never wanted to hurt them. Just like her, forced into this bleak world without a choice.
She became a Judge at the end because Dredd understood this. He doesn't change completely. But he understands Anderson's view point. Although unfortunately for Mega City One, Judge Dredd still views things in black and white. But he picked a good, real Judge to do some great counter thinking.
Boom. Dredd.