How did the narrator change when Mack died in Boys and Girls?
In Boys and Girls, Mack's death opened the narrator's eyes. She realized that Flora would be next, which forced her to realize the connection she had with the female horse. Flora, like her, was wild, free and did what she wanted. She didn't have the connection with Mack that she had with Flora, and once Mack was dead, she knew Flora would be next, because that was why they had the animals to begin with. It made the narrator not only relate to her, but feel bad for her.
Once her family decides it's Flora's turn, she doesn't want to take part in it, to the point where the narrator has the chance to stop Flora from leaving, but doesn't close the gate. When her brother tells on her, she knows her dad will never look at her in the same way, and she begins to accept the expected role of her gender in the time period.
Hope that helps! There is a lot more on this topic, but this should get you started. Check out the link for more information.