1. Uthman was not killed by only one individual so that he could be brought to book; rather an extensive uprising was launched an angry mob against him. The uprising led ultimately to his death. Imam Ali (AS) could not punish thousands of people for launching an uprising which Uthman himself was to blame for.
2. In confirmation of the above, it has been reported in The Event of Siffin that the narrator says: Abu Omamah Baheli and Abu Darda who were with Mu’awiyah in Syira met with the latter and said to him: “O Mu’awiyah, why do you fight this man (Ali bin Abi Talib)? By Allah, he is the one who first embraced Islam and he is the one who is more entitled to take over the government; he is closer to the Prophet of God (S), then on what basis are you fighting him?”
Mu’awiyah replied: “I am fighting him to avenge Uthman’s death because he has given refuge to Uthman’s murderers; tell him to let us get a hand on Uthman’s murderers and show them to us. In that case, I would be the first person from the Syrian people to vow allegiance to him.” 
The two men set off towards Ali (AS) and conveyed Mu’awiyah’s message to him.  Imam Ali (AS) said: “(If you are looking for Uthman’s murderers), these are the ones (those killers) whom you see.”
There were twenty thousand or more than twenty thousand people who were wearing armors with their faces covered in such a way that only their eyes were visible. They said: “We are all Uthman’s murderers, if they are seeking retribution of Uthman’s death, they should punish all of us.” 
Thereupon, Abu Omamah and Abu Darda returned and they never took part in any of the battles that ensued and this event took place in the month of Rajab.[1]
3. This question can be raised that if Mu’awiyah really intended to defend the oppressed, why did he not rise up to avenge the blood of Hamzah, the Prophet’s uncle?  Was it not because he and his father were involved in his death?  Was it not because he used Uthman’s death as an excuse to achieve his own political ends?
 

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