Complex sentence: Frustrated that Egan gang boss Tom Egan hadn't done anything to help his imprisoned brother, Harry went to the St. Louis police headquarters in December 1915 and offered to snitch on his gangmates in order to free Pudgy from prison.
Split into simpler sentences: Frustrated that Egan gang boss Tom Egan hadn't done anything to help his imprisoned brother. In December 1915, Harry went to the St. Louis police headquarters and offered to snitch on his gangmates in order to free Pudgy from prison.