The way the story read, drunk bastage hits his kids, he's overcome with grief and kills the man on the spot. After the fact, he comes to his senses, realizes what he's done and chucks his gun into the ocean, sewer, whatever.
What's the point of the penal code? It's to create social and monetary costs to crimes to either prevent them from ever happening or to punish those who do commit them. We know that in moments of intense grief people will do things that they wouldn't normally do, "temporarily insane" perhaps. Assuming this man had no prior violent record he's not likely to repeat this action. Also, given that he just witnessed his two sons get plowed over by a drunken idiot, I could see him being emotionally distraught to the point of actually meeting that temporarily insane marker, such that he's not going to have the capacity to worry about the potential punishment of his actions.
Punishing this man would serve no great purpose, you're not going to prevent such actions in the future.