Boo Radley's "history of violence" is more myth than reality. As Miss Maudie tells the children in Chapter 5, the stories about Boo are "three-fourths colored and one-fourth Stephanie Crawford," meaning they are a combination of superstition and fantasy. However, because he has been so repressed in his home, Boo could possibly have struck out at his father who refused to let him leave. After all, such an action is not uncommon for desperate people.
In his loneliness, Boo has made efforts to be in communion with the children, if only vicariously as he stealthily watches them. His mending of Jem's pants on the night when the children came to the windows in a dare and Jem tore his pants on the wire fence in flight from Mr. Radley's shotgun, along with his little gifts hidden in the knothole of the tree all indicate his efforts to establish friendship as well as the value that some kind of relationship holds for him.
So, while such a distanced relationship would not mean much to others, to the isolated Boo, his "friendship" with Jem and Scout is of paramount value. When he discovers that the children are threatened by the rapscallion Bob Ewell, Boo again becomes the desperate person who seeks to protect what he values greatly. For a shy recluse like Boo, there can be no efforts to talk Ewell out of his act that would be effective. Boo, in his panic and fear for the children that he must commit a desperate act--he stabs Ewell who is of no value to him whatsoever, while the children certainly are.