He was only reasonable, after being found out to be a fraud and liar by his peers. And that was not enough, he would still turn on you until he found out the cost of his Immortality is INSANITY. And that also was not enough until he realised all his peers turned away from him, and he would become an Insane IMMORTAL all by himself, rotting away for millennia. Then yes he finally became reasonable.
People like him, so much like Raislin Majere, only turned away from their path when they realised the costs to themselves personally. They would see every human burnt if they can reap benefits from it without a care or delay.