See, I wouldn't read that as 'Osborne overcame his fear of the dark'. I read it as 'the experience that actually first drove him insane'. It mentally unbalances him, but he's one of the quiet types of crazy, where it really doesn't start to come out until later triggering events set him off. This dovetails seamlessly into his modern behavior, where he thinks that 'willpower' can cure him from being crazy. He's /still/ just as insane as he ever was, just having suppressed it again.