Scout does not believe that Mrs. Radley died of natural causes, because she thinks that Boo killed her.
When Old Mrs. Radley dies, Scout and Jem are convinced that Boo Radley killed her. They have a romantically malevolent idea about Boo. He is the neighborhood phantom, supposedly locked in his house for years after attacking his father with scissors.
Although most people don’t pay much attention, Atticus goes to visit the Radleys and pay condolences.
Jem and I decided that Boo had got her at last, but when Atticus returned from the Radley house he said she died of natural causes, to our disappointment. (ch 8)
Scout and Jem had a much more romantic notion. They were convinced the monster Boo got her, and death from natural causes was too boring.
Most neighborhoods have a family that sticks to itself. Basically, there is no reason to doubt the Radleys. Yet they are different, and that makes the Finch children curious.
After Atticus departs for the Radley house—Atticus having just learned of Mrs. Radley's death—Jem and Scout surmise that "Boo had got her at last." In other words, they think that Arthur Radley has killed his mother.
Like many children, Jem and Scout have vivid imaginations. Furthermore, their imaginings are nurtured by the tales of gossipmonger Miss Stephanie Crawford, who lives across the street from the Finches. In one particular report of Boo Radley's supposedly fiendish deeds, Miss Stephanie states that Boo was once sitting on the floor as he clipped articles from The Maycomb Tribune. When his father passed near him, Boo jabbed the scissors he was using into the leg of his father. Boo then calmly pulled out the scissors, wiped the blood on his pants, and resumed what he was doing.
After Scout hears such a chilling tale about Boo, it is not surprising that she suspects foul play regarding Mrs. Radley's death. Mrs. Radley has rarely been seen outside her house, except for brief moments when she waters her cannas, a subtropical plant that has large foliage and vibrant flowers in red, orange, or yellow. Of course, the fact that the Radleys live in such seclusion leads to speculation amongst the adults and Scout's own suspicion of the Boo Radley's wrongdoing.