Complex sentence: During the final few years of his life, Guthrie was largely alone except for family: because of the progression of Huntington's, he was difficult to be around--common symptoms of HD include uncharacteristic aggression, emotional volatility, and social disinhibition.
Split into simpler sentences: During the final few years of his life Guthrie was largely alone except for family. These actions mirrored common symptoms of Huntington's including uncharacteristic aggression, emotional volatility, and social disinhibition.