When Lizabeth tells us, in retrospect, about that poignant moment when her chaotic mind led her to destroy the only beautiful thing in the horrid neighborhood, Miss Lottie's marigolds, she offers an insight of information that the reader must consider prior to being able to determine the extent to which Lizabeth hurt the old woman.
...she had nothing except a falling-down hut, a wrecked body, and John Burke, the mindless son of her passion. Whatever verve there was left in her, whatever was of love and beauty and joy that had not been squeezed out by life, had been there in the marigolds she had so tenderly cared for.
This clearly establishes two things. First, that Lizabeth has finally matured and sees the horrible act that she commits for what it is. Second, that she no longer sees Miss Lottie as the witch of the street like the other kids do. She sees the broken woman that perhaps Lizabeth herself will become. She sees a product of the terrible times; she sees Miss Lottie, finally, as what she really is: Lizabeth's victim, not her perpetrator.
After this statement, we learn that Lizabeth apparently tries to apologize. However, here is the telling part that shows how Lizabeth may have destroyed Miss Lottie's last attempt at hope.
I last saw her hut, completely barren at last, for despite my wild contrition she never planted marigolds again.
Clearly, Lizabeth's attempts to apologize could never make up for taking away someone's last gift of life. Nothing Lizabeth could ever do or say can fix a shattered illusion, or bring back a gleam of wishful thinking, as it was originally created. The marigolds were, by Lizabeth's own words, whatever was left of the beauty of love, joy and life that was not already squeezed out dry from the woman's body. It was all that Miss Lottie had left, and Lizabeth killed that.
Therefore, the fact that the woman never planted marigolds again, indicates that Lizabeth killed the last of the woman's attempt at life. Miss Lottie was a loner, odd, old, tired, poor, and had an invalid for a son. She had nothing else really to hold on to in life, but the flowers. Lizabeth did something worse than murder because she did not kill a human body; she destroyed the last vestiges of a tired soul. This is how she emotionally must have emotionally butchered the poor old woman.