Why did Prophet Muhammad (PBUH) order the killing of Ka'b bin Al-Ashraf?

The order to kill Ka'b bin Al-Ashraf was given by Prophet Muhammad (PBUH) in response to the harm that Ka'b was causing to the Muslim community. Ka'b was known for insulting the Prophet and inciting the disbelievers of Quraysh against him. This became particularly problematic after the Prophet migrated to Medina, where there was a mix of Muslims, idol-worshippers, and Jews. The situation was tense, as Ka'b's actions contributed to the hostility towards the Muslims.

In this context, a report states: Abdur-Rahman ibn Abdullah ibn Ka'b ibn Malik, whose father was among the three who repented, reported that Ka'b ibn al-Ashraf used to insult and incite the disbelievers of Quraysh against the Prophet (peace be upon him). When the Prophet migrated to Medina, its population included a mix of Muslims and disbelievers who worshiped idols, along with Jews who harmed the Prophet and his companions. Allah commanded His Prophet to be patient and to pardon them. The verse 'And you shall surely hear much that will harm you from those who were given the Scripture before you...' was revealed concerning them. When Ka'b refused to cease harming the Prophet, the Prophet ordered Sa'd ibn Mu'adh to send a group to kill him. Muhammad ibn Maslama led this party and narrated the story of the killing. When Ka'b was killed, the Jews and disbelievers were struck with fear and came to the Prophet, complaining that their leader had been killed. The Prophet reminded them of Ka'b's hateful words. He then invited them to pen a treaty with him, binding them to stop hostilities. The Prophet drafted a document between himself, them, and the Muslims collectively. Sunan Abu Dawud 20:73SunniSahih Isnaad (per Al-Albani)

This incident highlights the severity of Ka'b's actions and the impact they had on the Muslim community, leading the Prophet to take decisive action to protect his followers.