Why does Gene cause Finny to fall from the tree?

Gene, unlike Finny, has a nature that is corruptible through envy and suspicion; and, once inflamed, the bad side of Gene's nature takes him over, and causes him to harm his friend in a terrible way. The end of the chapter, with Gene causing Finny to fall from the tree, casts them almost as good vs. evil; Finny is cleansed of blame and shown to be pure of heart in this chapter, while Gene's character is revealed as being more pernicious than previously imagined.