The report is real, and it should not be softened into a mere literary dispute. But its strongest historical reading is that Ka'b ibn al-Ashraf was treated as an active hostile agitator during an armed conflict, not killed simply because he expressed a private criticism.
What the reports actually say
Sahih al-Bukhari records the Prophet Muhammad (PBUH) asking who would deal with Ka'b, describing him as one who had harmed Allah and His Messenger. Muhammad bin Maslama volunteered, and the Prophet permitted him to use misleading words to draw Ka'b out. Ka'b was then killed. Ibn Ishaq's account adds that Ka'b composed poetry against the Muslims, mourned their opponents after Badr, incited hostility, and went to Mecca seeking support against Muhammad and his community.
"Who is willing to kill Ka'b bin al-Ashraf, who has hurt Allah and His Messenger?"
Sahih al-Bukhari 4037
The strongest objection therefore has genuine force: this was a covert operation, and the permission to speak deceptively is not something one should hide. If Ka'b were merely a poet voicing disagreement, the killing would be an unjust execution of a critic.
Why "critic" is too narrow a description
The sources present Ka'b as more than a dissenter. He was a prominent political figure whose poetry was understood as mobilizing enemies, praising those who had fought the Muslims, and encouraging further conflict. The Muslim community was in a state of war, and the incident occurred in the shadow of the Battle of Badr and the subsequent confrontation with Quraysh. Classical historians and jurists accordingly treated Ka'b as a hostile instigator or combatant, not as an ordinary civilian publishing an unpopular opinion.
That does not mean every hostile poem would justify killing, nor that later readers must pretend the episode raises no moral difficulty. It means the decisive category in the report is hostile wartime action, not criticism as such. The Bukhari wording is brief, while the fuller historical context is found in works such as Ibn Ishaq's Sirat Rasul Allah.
What the deception does and does not establish
Islam condemns falsehood as a general rule. Yet the Prophet Muhammad (PBUH) also said, "War is deception," meaning that concealment and feints may be used against an enemy in war. This is not a license for everyday lying, fraud, or betrayal of a treaty. It is a rule of military conduct, subject to the wider prohibitions against treachery and injustice.
"War is deception."
Sahih al-Bukhari; Sahih Muslim
Thus, the incident does establish that a legitimate Muslim authority may authorize targeted action against an active enemy who is materially advancing a war. It does not establish a general Islamic rule that anyone who insults Muhammad must be secretly killed. Nor does it by itself provide a model for vigilantes, private individuals, or the suppression of peaceful disagreement. Those who cite it as proof of a universal blasphemy law have extended the report beyond what its historical setting and legal reasoning support.