Complex sentence: However, Yahiko's death in a fight causes Nagato to take over as the new leader of the organization, and to realize that peace can only be attained by teaching the world the pain caused by war.
Split into simpler sentences: However, Yahiko's death in a fight causes Nagato to take over as the new leader of the organization. This causes Nagato to come to the realization that peace can only be attained by teaching the world the pain caused by war.