Not insane. But you don't get to be a few thousand years old without picking up a couple personality quirks. Big D was very intelligent, very clever, but I can't help wondering if he had started to swing towards the idea that it was better to put faith in the Young Races, rather than rely on just the power of dragons and immortal elves. If he had lived longer, we very well may have seen him go 'rogue' himself, and start taking steps to make sure that the young races found out the truth.