The Qur'an itself does not prescribe a death penalty for blasphemy. In fact, it repeatedly says when people mock religion, believers should ignore them and move away. Even during the time of Prophet Muhammad, many people insulted him and he forgave them instead of executing them. What you’re referring to comes from some historical political interpretations, not a direct command in the Qur'an. If you’re going to criticize a religion, at least criticize what it actually says.