I'm going to tackle this answer in a few ways. The first is going to be cynical, and not personal thoughts/feelings.
Myers made the question of guilt or innocence ambiguous because he didn't want to push his reader in either direction. He didn't want to do that because he didn't want to offend anybody by saying one thing and having a group of readers be upset. I don't think this was Myers's intention, because by leaving it ambiguous, there will always be readers that are upset because it is ambiguous. That would be me. I really don't enjoy it when an author or movie director leaves those kinds of decisions up to me. I want to know. I'm not alone in this feeling either. Author Ray Bradbury even created a character with that mentality. Captain Beatty is all for burning books, because in his opinion they don't answer any questions. They leave readers with more questions than when they started.
Of course that is also why books are so fun to discuss. One reader can interpret an author's work one way, while another reader may interpret it completely differently. There often isn't a right or wrong answer either, because both readers can effectively defend their opinion using evidence from the piece of literature. I think that is why Myers left the question of guilt or innocence ambiguous. The book sparks discussion because both answers could be correct.
I also think that Myers left it ambiguous because that is more true to life. Not everything that every person ever does is always completely black and white. Life is full of "grey" areas. Do you cause harm to another person in order to save someone else from harm? Yes causes you to do harm. No means that you allowed harm to befall on someone that you could have helped. By leaving the question of guilt or innocence ambiguous, Myers is more accurately reflecting the real world in his fictional story.