What reason do the children have to be afraid of Mrs. Dubose in To Kill a Mockingbird?

In Chapter 11, Harper Lee introduces the overtly racist Mrs. Dubose. Mrs. Dubose is a cantankerous old woman who yells insults towards Jem and Scout as they pass her home. She is a mean person who immediately strikes fear in the children. After Jem loses his temper and destroys her camellia bush, he is forced to read to her every afternoon, except for Sundays, for an entire month. Scout follows her brother and stays by his side while he reads to Mrs. Dubose. Scout mentions that Mrs. Dubose's home had an "oppressive odor" which reminded her of an old, dilapidated house. Scout also mentions that Mrs. Dubose's face was the "color of a dirty pillowcase" and says that she drooled down the side of her liver-spotted cheeks. Mrs. Dubose also had pale eyes with knobby hands. Her mean personality coupled with her monstrous physical features scare the children.