Why would freethinking 'default' toward evil?
I think he was just being playful. People that refer to themselves as "Freethinkers" tend to be of the pagan persuasion, i.e., the rebellious towards God, as if anyone that believes in God have no thoughts of their own.
“"All science, even the divine science, is a sublime detective story. Only it is not set to detect why a man is dead; but the darker secret of why he is alive." ”G. K. Chesterton