Complex sentence: A year later, Harry returned for revenge on the two supervisors responsible for the accident, by killing them with his mining pick and cutting out their hearts, leaving them in decorative Valentine boxes and warning the town to never hold another Valentine's dance ever again.
Split into simpler sentences: A year later, Harry returned for revenge on the two supervisors responsible for the accident, killing them with his mining pick and cutting out their hearts, which he left in decorative Valentine boxes. He warned the town to never hold another Valentine's dance ever again.