Asma bt Marwan, the poetess of Yathrib was in habit of writing poetries that caused insult to prophet Muhammad. She was from the Banu Umayya and married Yazid b Zayd b Hisn al-Khatmi and was blessed with children.
I despise the Banu Malik and al-Nabit and Awf and I despise the Banu L-Khazraj
You obey a stranger, who is from other than you; not from Murad or Madhhij,
Do you have hopes of him even after the killing of your chiefs
Like one who awaits a well-cooked broth?
It was this poetry that made Umayr b. Adi b. Kharasha b. Umayya al-Khatmi swell in rage and he vowed “O Allah, indeed I promise that if you return rasul allah to Yathrib, I will surely kill her,” because Muhammad was at that time in Badr. After the return of rasul allah from Badr, Umayr b. Adi went hunting for Asma in middle of the night and entered her house. She was busy suckling her child and her children were fast asleep when Umayr threw the infant and pushed his sword into her heart forcing the blade to come from her back. After the murder he returned to say his dawn prayer (Salat al-fajr) along with Muhammad in Yathrib. After the salat the prophet turned and questioned Umayr “did you slay Asma the daughter of Marwan?” He replied honestly “yes, for you are dearer to me than my father, O Rasul Allah.” The reply made Muhammad burst in joy and replied “Two goats will not thrust about her.” This is the first time when someone heard such words from the mouth of Muhammad.
The Rasul then looked at the people present and told them that if you desire to look at a man who secretly helped Allah and his Nabi, then look at Umayr. Umar b. al-Khattab said “look at this blind man who is persistent in his obedience to Allah. Muhammad corrected Umar and said do not call Umayr blind but with one eye vision. The honor that Umayr got from Muhammad was quite serious in its nature; this gave extra boost to challenge the children of Asma who were busy burying their mother, that if together they fought Umayr he would slay them all.
The mercy of Muhammad was unquestionable, if this is the traits of Muhammad who failed to rebuke Umayr for the crime he did then we can understand that why Muslims react violently when Muhammad is criticized. The prophet of Islam cannot be a mercy to the mankind and those who follow his footsteps become violent and draw their swords just as Umayr did. The best example of mercy (Ar-Rahman) is Jesus, rebuking his disciples during his arrest by the Roman soldiers which could be read in (Matt 26:52), (Luke 23:34) and many merciful verses.
Al-Waqidi has mentioned the account in his book “Kitab-Al-Maghazi” but he shows no sympathy towards the atrocity but silences over the act and moves on.