A Sunni brother mentioned that...
A Sunni brother mentioned that: If Imam Ali disapproved of Uthman, then why did he risk the lives of his beloved sons, al-Hassan and al-Hussain, trying to protect the life of his adversary from the blood-thirsty rioters in Medina? According to the Shia sources, such news is dubious. We do not have any strong evidence that Imam Ali sent his sons to support Uthman’s House.
In fact, al-Tabari who is one of the important Sunni Historians said that Imam Ali deserted Uthman since Uthman insisted on keeping Marwan in his administration. Here is the related part from the History of al-Tabari, when the siege over Uthman was very severe: People informed Ali of the news.
Then Ali came to Uthman and said: “Surely you have satisfied Marwan (again), but he is satisfied with you only if you deviate from your religion and reason, like a camel carrying a litter that is led around at will. By God, Marwan is devoid of sense in regard to his religion and his soul. I swear by God, I think he will bring you in and then not send you out again. After this visit, I will not come again to chide you.
You have destroyed your own honour and you have been robbed of your authority.” When Ali departed, Uthman’s wife told him: “I have heard that Ali said to you that he will never return to you and that you have obeyed Marwan (again), who leads you wherever he wishes.” Uthman said: “What shall I do?” She responded: “You should fear God alone, who has no partner, and you should adhere to the practice of your two predecessors (Abu Bakr and Umar). For if you obey Marwan, he will kill you.
Marwan enjoys no prestige among the people and inspires neither awe nor love. People have only abandoned you due to Marwan’s position (in your councils). Send to Ali, then, and trust in his honesty and uprightness. He is related to you and he is not a man whom people disobey.” So Uthman sent to Ali, but he refused to come, saying: “I told him I would not return.” (3) Even if we suppose that Imam Ali protected Uthman in his last days, the protection was not because he loved Uthman to be in power.
He did so (if true) since he knew that this is a conspiracy, and he knew that those companions who plotted to kill Uthman, would become the avenger of his blood tomorrow, as it happened (e.g., the companions like Talha, Zubair, Muawiyah, and …) and it became a custom of the assassination of Caliphs with self-judgments including the assassination of Ali (AS) himself.