It must be considered that Uthman was not killed by a single person, whom can be sentenced to death by the Islamic ruler. Massive revolution by the Muslims, ultimately, led to death of Uthman. Therefore, Imam Ali (PBHU) could not punish tens of thousands of revolutionary people. A revolution that Uthman himself was not blameless in creating it.
Who is the name of the oldest martyr of Karbala (کربلا)?
Habib ibn Mazaaher / (حبیب بن مظاهر)
More info. about him:
In the name of Allah, the Entirely Merciful, the Especially Merciful
Briefly speaking, De facto, Othman was not murdered by just an individual, that he/she can be able to be punished by Islamic ruler, but there was a great revolution from Muslims that ultimately was led in murdering of him. As a result, Imam Ali (a.s.) couldn’t punish tens thousands of ...
At Battle of Karbala he was a small child and was with Caravan of Imam Hussain a.s. and he was inside the tent with the women and other children. Women especially Sayyedah Zeinab a.s. (the sister of Imam Hussain a.s.) were caring from children. All of the men with Imam Hussain a.s. who were supporting him were killed by army of Yazid and finally Imam Hussin ...