Can you please explain the details surrounding the killing of Ka'ab Ibn Ashraf? Is it true that he was assassinated by the Muslims?
I am sorry it would not be possible for me to give a detailed account of the circumstances in which Ka'ab Ibn Ashraf was killed. Books on early history of Islam and the Hadith works are a ready source of reference. However, I would like to allude to an important premise which has to be kept in mind while dealing with incidents which depict animosity with the rejecters of the faith of the time of the Prophet (sws).
While discussing this premise Shehzad Saleem writes:
In order to make the reward and punishment that is going to take place in the Hereafter an observable reality in this world, the Almighty selects certain personalities called Messengers to elucidate and explain the basic truths to their respective peoples. With the special help and assistance of the Almighty, they remove misconceptions that surround these concepts and vehemently remind people that if they do not accept these truths they shall be doomed in this world and in the Hereafter. Messengers give glad tidings of success both in this world and in the next to those who profess faith in them. People who deliberately deny these truths are punished in various degrees so that this whole episode can become an argument for the reward and punishment that is to take place on similar grounds in the Hereafter.
Keeping the above in perspective we see that among the enemies of the faith and rejecters of the Messenger of God were the polytheists, the people of the Book and the hypocrites which included some miscreants. These people tried to challenge the divine authority in the land and committed crimes against Muslims and even tried to kill the Messenger of God. When some of them started crossing all bounds the Almighty warned them:
As a consequence of this fundamental premise what needs to be understood is that some directives of Islam are specific to the age of the Prophet Muhammad (sws) and his Companions (rta) and cannot be extended to later periods. The reason for this is that a Muslim preacher cannot unveil the basic truths in their ultimate form as a Messenger is able to. Nor can he know whether a non-Muslim is knowingly denying it or not. Only the Almighty knows this. In the age of Messengers, He chose to reveal this judgement through His appointees, while in the second period of history He has not chosen to do so. Consequently, it is imperative that Muslims study the Qur'an and the life of the Prophet (sws), which are replete with the details of this divine scheme in the proper perspective.1
Ka'ab Ibn Ashraf fell in this category of the rejecters of the Messengers. He was a miscreant of the ultimate degree and posed a serious threat to Islam. Muslim historians have mentioned the extremities he committed and harms he inflicted upon the Prophet (sws) and his Companions (rta). He went as far as attempting to kill the Messenger of God. It was not the Prophet (sws) who decided that he be executed rather the Almighty Himself (see 33:60) who had already decided that the enemies of God and His Messengers who had not only denied the truth they were convinced of but had become brutal enemies of it had to be exterminated.
If the hypocrites and those in whose hearts is a disease and the agitators in the city do not desist, We shall most certainly set you over them, then they shall not be your neighbors in it but for a little while; (33:60)
1. Islam and Non-Muslims: A New Perspective, Renaissance, March 2002, pp. 65-6