Helen Keller was often frustrated because she could not effectively communicate. This frustration sometimes led her to do unkind or violent things. The word vicious means to do something mean or violent in a deliberate manner.
In Act One of "The Miracle Worker," Helen is playing with two children, Martha and Percy. While they are playing, Helen, "in a bizarre rage..., bites at her own fingers." Martha attempts to stop her, but "Helen topples Martha on her back, knees pinning her shoulders down, and grabs the scissors." Helen's mother comes to the rescue, but Helen fights her. This is just one example of Helen's violent behavior toward herself and others.
Later, Helen approaches the cradle where her baby sister sleeps and "unhesitatingly overturns it." The baby falls out, and their mother catches her. If the baby had not been caught, she might have died. Helen physically fights both her mother and Annie Sullivan in the first act. She also locks Annie in her room and hides the key.