Complex sentence: The mobsters motive seemed to be revenge on Dr. Hornbogen for the death of their friend, Fred Begeman, who had died of heart disease and kidney trouble under Dr. Hornbogen's watch.
Split into simpler sentences: The mobsters motive seemed to be revenge on Dr. Hornbogen for the death of their friend, Fred Begeman. Who had died of heart disease and kidney trouble under Dr. Hornbogen's watch.