In this particular episode of Criminal Minds (Season 3, episode 14- "Damaged"), Agent Rossi is intent on solving a 20 year old cold case of a double homicide in which three children found their parents murdered with an axe. We later learn that Joey, a "clown" at a carnival had followed one of the children the day before and wanted to play with her. He went to her house and accidentally entered the parent's room. The father attacked Joey with the axe and Joey got angry and accidentally killed them. His father protected him and cleaned up any evidence at the crime scene.
Also, in the episode, we learn some about serial killer Chester Hardwick who murdered at least 23 woman. He seemed not to care about his victims, but once he decided he was going to murder them, he stated that they "didn't have a chance." He even attempts to attack the FBI agents in the room that were interrogating him for a serial killer profile before he was to be executed on death row. He believed that if he killed them that he wouldn't be executed next week.
Both committed murders and are by definition criminals. That being said, it is not fair to compare Joey and Chester's crimes or categorize them the same. Joey is developmentally disabled and did not plan to commit the murders. He is bigger and stronger than he knows and his reaction to being attacked was to get angry and fight back. This resulted, sadly, in death to both of the parents, in which Joey instantly felt regret. However, Chester's murders were premeditated and he clearly didn't show remorse for his actions.
In terms of the law, however, both did commit crimes. Joey should be accountable for his actions. In a court of law, justice should be served. Simply because the criminal is developmentally disabled, doesn't mean he should walk free. However, the punishment should be less than Chester and be appropriate for his developmental age. Perhaps, a court would send him to an institution instead of prison.
Lastly, Joey's father should also be held accountable for protecting his son. Regardless of the circumstances, he aided a criminal and left three children without an answer to why their parents were murdered. The father should be tried in court and punished as per the law. It is the court's responsibility to punish the criminal, in this case, Joey, not the father's. However in most cases, the father, or any caretaker, should not be punished for the act of the crime itself. It is impossible to monitor and oversee someone's every move.