Why were the Boxers opposed to missionaries?

The Boxers were concerned about foreign influences and the introduction of opium by European traders. Adrian Hastings (A World History of Christianity) says the missionaries were haunted by the fear that pride would drive the Chinese to revolt, and so thought it important to keep them in a state of humility. Missionaries, Protestant as well as Catholic, had depended on the opium connection, with many travelling to China in ships carrying opium, their funds remitted via opium traders and their charitable or educational work relying on donations from resident Western firms connected with the opium trade.