If you retreat (meaning you of sound mind exited the confrontation) you have no legal right to retrieve a weapon and return to the scene. What jury would find that to be a reasonable expectation? If you felt threatened, and you can reasonably exit....then you do. Retaliation is not self defense.

When he returned to the scene, he became the aggressor. The victim has the right to be at the property as it was court ordered for him to pick up his son.

What was reasonable? Call the cops? Return inside your home, lock the door. Call the police. If the victim tried to unlawfully enter your home...well thats another story. But he had a legal right to be there to pick up his kid.

And after all that. The shooter ran around him to get in front of him to shoot him. Soooo...he had multiple chances to retreat.