A lot of Christians were concerned with whether the Christian allegory in The Lion, the Witch, and the Wardrobe would survive the translation to film. Time ran an article titled, "How to Tell if The Lion, the Witch and the Wardrobe is a Christian Film." (Link via open book.) The author wrote that if this sentence by Aslan, along with three others by the White Witch, made it into the released movie, that would "constitute a kind of evangelical sniff test":
The Witch knew the Deep Magic. But if she could have looked a little further back... she would have known that when a willing victim who had committed no treachery was killed in a traitor's stead, the Table would crack and Death itself would start working backwards.
Barb Nicolosi, who attended a preview of the movie, assures us, "All the lines the Christians are worrying about are in there."
So let me say that I think the "willing victim who had committed no treachery" line is the weakest bit in the entire book.
Yes, yes, it's the key to the story as Christian allegory, and without it Aslan wouldn't be a Christ figure so much as one of those magical lions that pop right back up after being killed. But it's a glass key, and if you aren't careful it will break right in your hand.
To take just two points: First, this "Deeper Magic From Before the Beginning of Time" makes the "Deep Magic" according to which the White Witch may kill any traitor an arbitrary and passing thing. Tough luck for the traitor right before Edmund, who gets it on the Stone Table; good luck for the traitor right after Edmund, who not only doesn't have the White Witch on his tail, but who doesn't even have to feel bad about anyone dying for him.
Second, the Deeper Magic seems to work for anyone. It's not who Aslan is that breaks the Stone Table. In a sense it's not even what he does, but what he knows. At any time, some mother Vixen might have offered to die in place of her son, and hey presto, the White Witch would have been out of business. And in any case, her power was already breaking before she agreed to kill Aslan. Aslan basically tricked her into making a deal that would ensure her own destruction.
So yeah, that the innocent dying for the guilty can be an act of great power is a Christian notion. But as it's found in The Lion, the Witch, and the Wardrobe, it's a pretty feeble version (also shown by the fact that it can be excised altogether by changing one single sentence), and Christians might be better off not pressing it too hard.
All this assumes C. S. Lewis meant the story to be allegorical. Other books in the Chronicles of Narnia, however, show that Aslan is not merely a Christ figure, he is supposed to be the Second Person of the Trinity Himself. That's a step, not mandated if we limit ourselves to this one book, that turns the Deeper Magic from a weak allegory to a cover-your-eyes awful theology.