ISLAMABAD—Malala Yousafzai, a teenage campaigner for girls' education who was nearly killed by Pakistani militants, was feted at the United Nations last week. Here at home, however, she has been widely portrayed as part of a Western conspiracy against Islam and the developing world.

A 1,800-word open letter in imperfect English by Adnan Rasheed, one of the most feared Taliban leaders in Pakistan, outlined these conspiracy theories Wednesday, describing the type of secular education that Ms. Yousafzai championed as "satanic"...