As mentioned elsewhere, the primary reason why Uthman was killed was for mismanaging the affairs of the Muslim nation, and it was Lady Aishah who was the forerunner to censure him regarding this.
While both the Prophet and Abu Bakr had distributed the revenue from the treasury equally amongst the Muslims, Umar, on the other hand, begun to shift the distribution in favor of his allies. Thus Umar had assigned Lady Aishah, the daughter of Abu Bakr; Hafsa, his own daughter; and Umme Habiba, the daughter of Abu Sufyan with 12,000 dinars a month, while providing the rest of the wives of the Prophet with only 5,000 dinars per month.
When Uthman came to power, he wanted to reinstate the equal stipend among the Prophet’s widows but Lady Aishah protested, and thus the campaign to turn against Uthman first began by her. According to al-Tabari, she is recorded to have said, “Kill this Nathal (the nickname she gave to Uthman), for he has disbelieved.”
Besides his practice of nepotism and favoritism, Uthman on many levels suspended the practices of the Qur’an and the Holy Prophet. Blatantly disregarding the two most fundamental aspects of Islam had undoubtedly caused a furor amongst the Muslim population, as a result of his actions, Uthman was murdered and his corpse was prohibited from being buried inside the Muslim cemetery of al-Baqi, and to add to this, the prayer of the deceased was prohibited from being performed on him.
Further information about these individuals can be found in the history of al-Tabari.
 Tarikh al-Yaqubi, 2:132
 Al-Tabari, 5:72
 Nathal was a Jewish man with a long beard who resided in Madinah at the time.
 Tarikh al-Tabari, 3:477; Ibn A’atham, Al-Futuh, 1:64
 When Mu’awiyah came into power, he extended the boundaries of al-Baqi cemetery to include the burial site of Uthman.
 Tarikh al-Madinah al-Munawarah, 3:1052
 Tarikh al-Tabari, 2:452