Whenever critics and missionaries want to spread lies about Islam, they write an article trying to show that Mohammed was violent for ordering someone to be killed – they portray the persecutor as the victim and the one persecuted as the aggressor.
This is the case of Amr ibn Jihash. Amr ibn Jihash tried assassinating Prophet Mohammad (p), hence when he found out what Amr ibn Jihash was going to do, the Prophet (p) left for safety and had later prepared his companions to prepare for war against the aggressors who had planned to assassinate him. In short, Amr ibn Jihash is not innocent as missionaries make him out to be.
THE DEPORTATION OF THE B. AL-NADIR, A.H. 4
According to what Yazid b. Ruman told me the apostle went to B. al-Nadir to ask for their help in paying the bloodwit for the two men of B. Amir whom Amr b. Umayya al-Damri had killed after he had given them a promise of security. There was a mutual alliance between B. al-Nadir and B. Amir.
When the apostle came to the about the bloodwit they said that of course they would contribute in the way he wished; but they took counsel with one another apart, saying,
‘You will never get such a chance again. Who will go to the top of the house and drop a rock on him (T. so as to kill him) and rid us of him?’
The apostle was sitting by the wall of one of their houses at the time. ‘Amr b. Jihash B. Ka’b volunteered to do this and went up to throw down a rock. As the apostle was with a number of his companions among who were Abu Bakr, Umar, and Ali news came to him from heaven about what these people intended.
So he got up (T. and said to his companions, ‘Don’t go away until I come to you’) and he went back to Medina.
When his companions had waited long for the prophet, they got up to search for him and met a man coming from Medina and asked him about him.
He said that he had seen him entering Medina, and they went off, and when they found him he told them of the treachery which the Jews meditated against him.
The apostle ordered them to prepare for war and to march against them (670), Then he went off with the men until he came upon them (680). 
Kitab Al-tabaqat Al-Kabir – Ibn Sa’d:
They said: O Abu al-Qasim, we will do as you like. Then some of them went into secret consultation and decided to act treacherously against him (Prophet). Amr Ibn Jihash Ibn Ka’b Ibn Basil al-Nadari said:
I shall ascend on the roof of the house from where I shall throw a stone down (on him). Thereupon Sallam Ibn Mishkam said: Do not do it. By Allah, he will come to know what you intend t do with him. It would be a violation of our agreement with him. The Apostle of Allah, received information of what they intended to do. 
 Ibn Ishaq – Siratu Rasulillah – (Translator: A. Guillaume), page 437
 Kitab Al-tabaqat Al-Kabir by Ibn Sa’d, volume 2, page 69