system
Given the following post, respond as if you're commenting on Reddit:
human
[12 angry men] But who killed the kid's dad?

It was someone who had the same model knife and stabbed him the exact day that the kid lost his knife. or someone who had access to the kid's knife

also, it was someone who could pass for the kid in front of a woman who may not have (but may have) been wearing her glasses. 

Who could have done this?
gpt
Regarding the knife, it was suggested (by the juror going out and easily buying one) that it was exceedingly common and easily acquired.

The racism of the period (again, thoroughly demonstrated in text and movie) suggests the identification of the suspect was of the 'they all look alike,' variety, so almost any non-white male could've been the actual attacker. 

Based on what we can infer, it was likely a random crime of opportunity, such as home invasion for theft or a breaking and entering gone wrong, that lead to the fathers death.  Due to the lackluster efforts of the police, it's likely the actual perpetrator would never be found.