Complex sentence: Winters told the men to wait for his command to fire, but Guarnere was eager to avenge his brother and, thinking Winters might be a Quaker and hesitant to kill, opened fire first, killing most of the unit.
Split into simpler sentences: Winters told the men to wait for his order to fire, but Guarnere was eager to avenge his brother. Claiming he thought Winters might be hesitant to kill, he opened fire immediately, killing most of the unit.