Lizabeth destroys the marigolds to release the anger/frustration she feels about her life. After Lizabeth hears her father crying, Lizabeth belives "I too could cry and be comforted." Sadly Lizabeth has no source of comfort, so she lashes out on the marigolds to show her anger.
Hope this helps, have a BLESSED and wonderful day! :-)
-Cutiepatutie <3