There was a spot on his mind that was not sane...

"Being surrounded with every conceivable kind of revolt from
infancy, Gabriel had to revolt into something, so he revolted
into the only thing left--sanity. But there was just enough in
him of the blood of these fanatics to make even his protest
for common sense a little too fierce to be sensible. His
hatred of modern lawlessness had been crowned also by an
accident. It happened that he was walking in a side street at
the instant of a dynamite outrage. He had been blind and deaf
for a moment, and then seen, the smoke clearing, the broken
windows and the bleeding faces. After that he went about as
usual--quiet, courteous, rather gentle; but there was a spot
on his mind that was not sane."

G.K. Chesterton, "The Man Who Was Thursday"