Well if you read about ‘Asma’ bint Marwan, Abu Afak, Ka’b ibn al-Ashraf](Ka'b ibn al-Ashraf - Wikipedia), Abu Rafi’ ibn Abi Al-Huqaiq, who were killed for opposing the prophet with poetry, also Khalid bin Sufyan who was killed for the sin of incitement, Banu Nadir who were invaded for criticizing the prophet, you might change your opinion. There are other examples too that convince the Sharia scholars to be convinced that insulting and criticizing the prophet is something to kill for.