In "The Ball and the Cross" where did McIan go wrong?

In the G.K. Chesterton's _The Ball and the Cross_ there's a point where it's obvious that Chesterton doesn't think Evan McIan's initial philosophy, if taken to its fullness, would bring about peace on Earth. What exactly was it about his philosophy that left him open to temptation (in his dream) and how did his viewpoint change. Furthermore, what was it about seeing the old man that made him want to stop fighting? I ask this as a Catholic who feels compelled to smash things occasionally, but somehow feels it's not the right thing to do!