Complex sentence: Less than three years later, McGurn, by then impoverished and abandoned by his fellow gangsters, was assassinated by three men using machine guns on February 15,1936, the seventh anniversary of the St. Valentine's Day massacre.
Split into simpler sentences: Less than three years later, McGurn, by then impoverished and abandoned by his fellow gangsters. He was assassinated by three men using machine guns on February 15,1936, the seventh anniversary of the St. Valentine's Day massacre.