Lizabeth feels horrible to hear her parents fight, and she knows that her family is struggling in the Great Depression. This causes her to take her anger out on the marigolds. In the beginning of the story she acts very childish and immature, as she doesn't realize the marigolds value. She already threw pebbles onto the flowers but now she was angry enough to completely destroy the marigolds.
Lizabeth destroys Miss Lottie's marigolds because they represent happiness which her and her family do not have. She is not happy so she doesn't think Miss Lottie deserves happiness either. She starts to throw away her childish ways after realizing the beauty of the flowers. Destroying the marigolds would be her last act of childhood.
Lizabeth makes eye contact with Miss Lottie and realizes what she has done. She realizes Miss Lottie has feelings too and is regretful of destroying the only happiness Miss Lottie had. She becomes more mature and adultlike after learning her lesson. Lizabeth plants new marigolds in the garden after Miss Lottie passes away.