What is #11's point about the defendant's panic?

11th Juror raises the question of why the boy would return home, several hours after his father had been murdered, if he had been the one to murder him. 4th Juror suggests that he left the knife in a state of panic and then decided to come back for it later, but 11th Juror challenges this by reminding him that the fingerprints had been wiped off the knife, suggesting that the murderer left calmly.