How may we associate the narrator's action of helping Flora escape with her witnessing the shooting of Mack?

Mack is an old horse, and it's common practice on farms for old horses to be taken out and shot when they're no longer deemed useful. Sadly, this is what happens to Mack. His meat is to be used to feed the foxes, and so he won't be the last horse to suffer this fate. Having witnessed Mack's shooting at first hand, the narrator suddenly has a realization of what death involves. Growing up as a little girl on a farm, she'd previously been sheltered from the harsh realities of death, but curiosity gets the better of her and she chooses to watch her brother and father shoot the old nag to death.
The narrator doesn't want Flora to suffer the same fate as Mack so she opens the gate as wide as she can to help the young mare escape. She doesn't quite understand why she's disobeying her father in doing this, but really there's nothing very complicated about her actions. She's been fantasizing about becoming a hero and a rescuer, and helping Flora to escape means that she can live out those fantasies. But there's little doubt that, had she not witnessed Mack's death, then she wouldn't have had the courage to defy her father and open the gate to facilitate Flora's bid for freedom.