Shariah law and the shooting of Malala

According to the Taliban, Shariah law says that even a child can be killed if he is propagating against Islam. And hence they shoot Malala Yousafzai, the Pakistani schoolgirl, aged 14. How can adults be so blinded to rational thought and shield their ghastly deeds and justify their heinous behaviour by hiding behind any such so-called ‘law’. And if Shariah law does indeed say such a thing, why do responsible Muslims not critique and condemn it?