Chapter 1 of Harper Lee's novel To Kill a Mockingbird contains the story of why Mrs. Radley runs out of the house and into the street, "screaming ... that Arthur was killing them all."
According to the story passed from Miss Stephanie to Jem to Scout (the narrator), Arthur "Boo" Radley out of nowhere stabs his father in the leg with a pair of scissors and then goes right back to what he was doing beforehand. This incident supposedly was one of the events that eventually leads Mr. Radley to virtually imprison his son in his own home.
Above, I wrote "according to the story" and "supposedly" because I'm not sure that we're supposed to take the story of the scissors-stabbing as absolute truth. It may have some foundation in truth, but I imagine that the tale has grown a bit and become a bit more lurid even time it has been retold.
This episode that you are talking about happens in the first chapter of the book. This happened sometime after Arthur (Boo) Radley had been locked up by his father, but at a time that Jem is able to remember.
What happens is that Boo stabs his father. He just takes a scissors that he was using to cut stuff out of a newspaper (for his scrapbook) and stabs his father in the leg. He doesn't make any move to go after his mother, but she runs out screaming that Boo is killing them all.
I just finished the book and got an A on the test and the answer to your question is very simple. Arthur ( Boo ) stabbed his father in his thigh with scissors and then continued on cutting his scrapbook decorations. They thought he was crazy or mental.