All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
Not all the Muslims in Egypt were hostile to ‘Uthmaan . Rather, he was killed by a small band from the people of Basrah, Kufah, and Egypt. The reason for their hostility was their objection to some of his actions and a number of claims and accusations that they raised against him. Some of their claims were pure lies, and some were against decisions made by ‘Uthmaan based on his Ijtihaad (personal reasoning) which he is rewarded for (be it correct or incorrect). Scholars refuted these claims well and clarified that those who disobeyed him and killed him were wrongful and impious. To know more about these claims and accusations and their refutation, you may refer to the book entitled Huqbah min At-Tareekh by Shaykh ‘Uthmaan Al-Khamees.
Allah knows best.