When 'Uthmaan, radyAllaahu 'anhu, was killed, the Muslim nation did not see anyone possessing more right to succeed him other than 'Alee, radyAllaahu 'anhu, so they pledged allegiance to him. So those who sought to turn the people away from him set about their goal by manifesting the outrageous and scandalous murder of 'Uthmaan. And it was just as they said it was. But then they added to it that the one who conspired his murder and carried it out was none other than'Alee, radyAllaahu 'anhu. And this was a lie and a slander against him!
And 'Alee, radyAllaahu 'anhu, would swear and reaffirm his oaths in denying this accusation - and he was truthful and innocent in his oath, may Allaah be pleased with him. But they began to fight against him, claiming that their struggle was for the sake of the Religion and that it was pleasing to Allaah, and then they began to fight with his children. These individuals strove hard in publicizing this (lie), propagating it on the mimbars on the days of Jumu'ah, as well as on other occasions in which there were large gatherings. This continued until it settled into the hearts of their followers that the matter was as these individuals said it was, and that the tribe of Marwaan had more right (to the Khilaafah) than 'Alee and his children due to their closeness to 'Uthmaan, and that they had more right to avenge his, radyAllaahu 'anhu, death. So in doing this, they were able to unite the hearts of the people against 'Alee and his sons and to turn the people to fight against him and his children after him. This asserted the kingship for them and their rule became established as a result of that.
While in privacy, one of them would say to those he confided in, something with the meaning: "No one amongst the Companions was more restrained from (causing harm to) 'Uthmaan than 'Alee." So it would be said to him: "Then why did the people revile him?" So he would respond: "The kingship (i.e. end of the Khilaafah) would not be established if it weren't for that."
The meaning of this is that if they did not turn the people's hearts away from 'Alee, radyAllaahu'anhu, and his children, and if they didn't attribute the injustice done to 'Uthmaan to them, the hearts of the people would not feel sympathy for them (later), due to what they knew of their beautiful attributes and honorable qualities, for they used to rush to follow them and pledge allegiance to them (in the past). And because of this, the Umayyah dynasty came to an end and the people ceased obeying them.
Taken from the book: The Difference between Advising and Condemning by the Imaam, the Haafidh Zayn-ud-Deen Ibn Rajab Al-Hanbalee (rahimahullaah) [d. 795H]