Especially with the whole, "No, we're ALL staying, Peter" thing that wasn't there in the book, complete with explanatory speech.
And he really didn't have a clue what he was getting into until he'd at least spoken to Lucy after getting into Narnia on his own once, so I'm personally inclined to call everything up to that point simple human error (whether or not it counted towards his label as traitor is another matter, but he didn't understand what he might be dealing with before then).
Maybe it's from getting too much into the allegory and trying to make the Biblical stuff perfectly mesh with the Chronicles. Which makes Edmund the Judas figure even though he's overall closer to Saul/Paul in light of the entire character arc he goes through.
And that probably where some of the issues with Susan come through, too, among those who have read all the books. The one not there at the end HAS to be fated to forever not be there, even though her ultimate fate is still going to be up in the air for possibly six full decades of time during which she still has free will and so forth. And that's disregarding the question of whether she would be with the others at the end of the book if she had been with her parents during the book, which is a rather different question than the one the book answers, even though both seem to be treated as the exact same thing most of the time. Grr.