Why do you think Faulkner does not directly describe the burning of de Spain's barn? And the shooting of the boy's father?
I think he does not show the burning of the barn because the actual event isn't what's crucial to the story. Something like that is exceptionally vivid, and would draw the eye/the attention. Faulkner's focus in the story is on how people react—individually and as a community. It's also about weighing accounts of what happened and explanations for the way the world is. That's why you get comments like "Barn burner!" That's the label the world applies, just as the boy is named " Colonel Sartoris Snopes." The words matter, and the people have to sort out what they mean.
Faulkner doesn't discose weither or not Abner Snopes is killed or escapes because it is irrelevant to the conflict of the story. The confict being that Sarty wants to believe his father is a good and honorable man. When in reality he isn't. When Abner goes to burn the second barn it is over in Sarty's mind that his father can change or that he is the man he wants his father to be. Either way the father now dead to him in that sense. Even if the father escaped he can't return to his family.