A good book to read on this is The Succession to Muhammad [s] by Wilferd Madelung, who explains very clearly the subtleties of the political situation of 'Uthman. 'Uthman had incited fury among Egyptians, Kufans and others because of his nepotism and awarding his family property that belonged to the umma. He had also cut 'Aisha's stipend, angering her. The Egyptians came as a delegation to 'Uthman with their grievances, but he refused to listen to them. Instead, he appealed to Imam Ali (as) to tell them to go away. Imam 'Ali (as) warned 'Uthman more than once that in order to calm the situation, he must listen to their grievances and correct his actions. 'Uthman refused to listen, and so Imam 'Ali (as) distanced himself. 'A'isha sent out letters inciting those who were angry with 'Uthman to kill him. 'Uthman's palace was stormed. Interestingly, 'Uthman was abandoned by his cousin Marwan and his sons in his time of need. Imam 'Ali (as) sent just Imam Hasan (as) (not Imam Husayn) and some others to try to calm the crowds, but they went ahead and killed 'Uthman. Because Imam 'Ali (as) had not directly intervened, Aisha later took advantage of this and accused him of being responsible for 'Uthman's murder. She used this allegation to try to overthrow Imam Ali (as). Imam 'Ali (as) says in sermon 30 in Nahj al-Balagha that both parties were in the wrong: 'Uthman was wrong for misappropriating property and governing badly; and the Egyptians and others were wrong for murdering 'Uthman, basically meaning that the grievances should have been addressed through a legal process: "If I had ordered his assassination I would have been his killer, but if I had prevented others from killing him I would have been his helper... I am putting before you his case. He appropriated wealth and did it badly. You protested against it and committed excesses therein. With Allah lies the real verdict between the appropriator and the protestor.' Thus, in sending Imam Hasan (as) to try to calm the crowds, Imam 'Ali (as) was not actually siding with 'Uthman, nor supporting him, but was rather trying to prevent excessive and unlawful behaviour on behalf of the aggrieved parties.