Very strange. I get the opposite conclusion from the people he ordered killed for criticizing him. Here's one example.
From Ibn Sa`d's Kitab al-Tabaqat al-Kabir, translated by S. Moinul Haq, volume 2, pages 30-31.
SARIYYAH OF `UMAYR IBN `ADI
Then (occurred) the sariyyah of `Umayr ibn `Adi Ibn Kharashah al-Khatmi against `Asma' Bint Marwan, of Banu Umayyah Ibn Zayd, when five nights had remained from the month of Ramadan, in the beginning of the nineteenth month from the hijrah of the apostle of Allah. `Asma' was the wife of Yazid Ibn Zayd Ibn Hisn al-Khatmi. She used to revile Islam, offend the prophet and instigate the (people) against him. She composed verses. Umayr Ibn Adi came to her in the night and entered her house. Her children were sleeping around her. There was one whom she was suckling. He searched her with his hand because he was blind, and separated the child from her. He thrust his sword in her chest till it pierced up to her back. Then he offered the morning prayers with the prophet at al-Medina. The apostle of Allah said to him: "Have you slain the daughter of Marwan?" He said: "Yes. Is there something more for me to do?" He [Muhammad] said: "No. Two goats will butt together about her. This was the word that was first heard from the apostle of Allah. The apostle of Allah called him `Umayr, "basir" (the seeing).