I think that the author wanted to call our attention to our natural tendency to accuse and mistrust one another. Throughout the novel, the characters are constantly questioning each others' motives and actions. Granted, they are trying to find out who a murderer is. What's interesting is that despite all of them being ordinary people, they very easily came to believe one of them could have killed Sam Westing.
Greed is what ended up motivating the characters to play the game. They all could have found a use for the money. The money motivated them to point fingers at each other. The only one who could really be considered innocent was Turtle (despite her tendency to kick people in the shins). As a child, she did have an innocence and altruism about her. She was the most trustworthy. She kept people's secrets, including Sam Westing's.