If blasphemy was punishable by death in Islam, then the Prophet would have been the first one to order the killing of hundreds of his foes who later became his closest companions.
With the exception of a very few earlier Arabs who accepted the Prophet as the Messenger of Allah, the majority of people of Makkah opposed him, humiliated him, cursed or blasphemed him or even tried to kill him, yet he preferred to practice forgiveness and to seek the divine mercy for them.
The old woman who used to throw garbage on the Prophet was visited by him when he did not see her throwing it anymore to learn that she was not well.
When Suhail bin Amr, a poet who composed poetry blaspheming the Prophet was taken as a prisoner of war after the battle of Badr, the Prophet asked his companions to show kindness to him.
There are examples after examples to prove that the Prophet never resorted to violence against those who were showing utter disrespect to him.
The killing of Deborah is a stab in the heart of Islam and a humiliation of the Prophet by those who claim to be his followers.
Those who are supporting her killing or similar actions are the worst enemy of Islam who neither understand Islam nor respect the Prophet. No matter who they are, they must be challenged on the basis of the Quran and the teachings of the Prophet.
Unfortunately, their argument is built on a position that is supported by a good number of Muslim religious scholars all over the world that demand death for acts of apostasy and blasphemy, two of the practices that were developed under the influence of corrupt despotic Muslim rulers.