Why was 1962 film “The Miracle Worker” marketed as “terrifying”? (Poster)

I recently went to a museum exhibit showcasing original 1960’s movie posters, and was very confused by the poster for “The Miracle Worker”. I haven’t seen the movie since I was a kid, but isn’t this a pretty heartfelt drama? Why does the poster look like the movie is “The Exorcism of HELLen Keller” (sorry… I had to)?

There was also a newspaper advertisement from that year that calls the film “the most beautiful and terrifying film of the year”. I remember there being some more intense scenes showing Keller’s physical outbursts and violence, but advertising the movie as “terrifying” seems so strange! Any ideas?