Movie's argument in defense of criminals / criticism of death penalty
The two main characters kill an innocent family of four, and they are accordingly tried, convicted, and executed. Society believes that these two are scum.
But the movie's argument is that these two are not scum. They are products of poor environment and insufficient rehabilitation / treatment efforts.
The biggest problem is that both Dick and Perry endured a tough upbringing, with bad parents. Dick came from a poor family (see the place where he and his dad live). As a result of coming from a poor family, he gets a fixation with stealing, as he believes money is everything in society (e.g. he talks about how rich people would never get electrocuted for a crime, like poor people do). In addition, there's evidence that Dick's dad neglected him. When the agents talk to him, Dick's dad talks about how Dick is a nice boy who innocently enjoys hunting and such; those sort of statements show just how much Dick's father didn't know him.
Likewise, Perry suffered trauma from witnessing his father abuse his mother. The numerous hallucinations indicate the Perry has severe mental illness. That he got through parole with this mental illness shows a significant flaw in the system: the system should have found his mental illness. In addition, Perry's dad doesn't appear to be all there when the agents talk to him. He literally hadn't spoken to Perry in 5-6 years, and he didn't know that Perry was out on parole. Hence, Perry's dad was likely neglectful.
One very significant detail is that, right before Perry unleashes and kills the family, Perry sees an image of his father. The meaning is that the mental illness is the cause of Perry's decision to murder that family.
Overall -- neither Perry nor Dick are really cold-hearted. Dick, during the executions, doesn't do the killing; Perry is the shooter . Perry even hands Dick a knife, and Dick finds himself unable to do anything with the knife. That reluctance shows that Dick isn't a murderer. That he chooses Perry for a partner shows his awareness that he can't bring himself to kill people. The significance, again, is that Dick isn't a murderer.
Likewise, though Perry commits the murders, the mental illness really prompts him to commit the act, as he sees his dad right as he shoots the family. Plus, Perry does show a conscience on many occasions - he takes the father down because the father feels cold, he gives the father the glasses when the father couldn't see, he prevents Dick from harassing the daughter etc.
^ On that basis of the above, I believe the movie tries to make people less judgmental about criminals. Accordingly, the movie attacks capital punishment as cruel and inhumane.