This article has a little more detail:
Grieving father charged in shooting death of driver
“No one recovered a gun at the site of the crash, but people reported hearing gunshots fired after the accident.
Brazoria County investigators interviewed several witnesses who were present at the accident scene. Those witnesses said they observed David Barajas walk away from the accident site to his home and return to the area before approaching Banda’s car when the gunshots were fired.”
Alright, so this wasn’t some months-later vigilante justice. The dad watched his kids killed right in front of him by an obviously drunk guy, walked straight to his house, got the gun and shot the dude right there.
So no premeditation. Temporary insanity. This changes the whole story.