I think the reason Taliban targeted a young girl like Malala was to stop her from spreading ideas of democracy, civilized society, and people’s liberty and rights. They feared about the spread of education, the advocation of women's rights, and the adoption of western culture because all these would damage their control over people.
Malala and her speeches became more influential over time, so the Taliban wanted to murder her to set an example to other social activists. They tried to use Malala’s death to intimate the others and prevent them from further threatening Taliban ruling in this area. Taliban’s action only proved that it was a terror organization that could not hear any different voices and used killing to silence people. Taliban