Complex sentence: Realizing the stone will allow him to destroy Xandar without Thanos' help, Ronan reneges on their deal and sets the stone within his war hammer, becoming an unstoppable force.
Split into simpler sentences: Realizing the stone will allow him to destroy Xandar without Thanos' help, Ronan reneges on their deal. Setting the stone within his war hammer, Ronan becomes an unstoppable force.