Complex sentence: Karl realizes that this means that he will be taken by the White Walkers, and that this is how Craster protected himself from them, and reasons that if they continue to uphold the bargain they too will be safe.
Split into simpler sentences: Karl realizes that this means that he will be taken by the White Walkers, and that this is how Craster protected himself from them. He reasons that if they continue to uphold the bargain they too will be safe.