Deception does function in each of these stories, but the resulting themes are different. Macbeth (and Lady Macbeth) deceives everyone (except the witches) in efforts to gain and then sustain power. Their deception is motivated by greed and later their fear and guilt are what motivates them to continue that deception.
Gene’s deception of Finny, and everyone else, is less conspiring. When he pushes Finny off the branch, it is a conscious and subconscious event. Gene continues the deception for reasons similar to Macbeth. He is afraid of getting caught and he feels guilty about it.
With Macbeth, the deception is motivated and sustained for sinister reasons: greed, power, fear and guilt. In A Separate Peace, Gene is motivated by jealousy and maybe even teenage anxiety. His deception is clearly less sinister than Macbeth’s. Gene later discovers that he is not jealous of Finny’s power and self-confidence. He is jealous of Finny’s innocence. Gene is annoyed that Finny won’t grow up and acknowledge the war. I’m not saying we should give Gene a pass on his transgressions. But if you’re comparing Gene and Macbeth, with regard to deception, you have to note that Gene is dealing with a natural resistance to the harsh reality of adulthood and Macbeth is dealing with an infatuation with power.