In Things Fall Apart by Chinua Achebe, would it be right to condemn Umuofia for the killing of Ikemefuna ?

An interesting question! According to the text, Okonkwo was the one who struck the killing blow during Ikemefuna's execution. Prior to Ikemefuna's death, Ogbuefi Ezeudu, a village elder and former warrior of Umuofia, had warned Okonkwo against participating in the boy's execution.
Ogbuefi's reason for discouraging Okonkwo from slaying Ikemefuna was that the young boy looked to Okonkwo as a father figure. In Ogbuefi's eyes, Okonkwo's participation would be dishonorable. Even Obierika later proclaimed that what Okonkwo had done would not "please the Earth. It is the kind of action for which the goddess wipes out whole families."
Yet, despite the fact that two clansmen disapproved of Okonkwo's participation in Ikemefuna's death, the tribal leaders still subscribed to the Oracle's authority in the matter. Ikemefuna was slated to die, regardless of anyone's wishes in the matter. Despite the judgment, there were those who disagreed with the execution of the innocent boy. It appears, though, that the larger community had no choice but to submit to the authority of the Oracle and its tribal leaders.
The text tells us that Nwoye was especially traumatized by Ikemefuna's death, a death he had been powerless to prevent. Additionally, while Nwoye's mother sympathized with Ikemefuna's predicament, she too was powerless to affect a rescue on his behalf. It can be seen from the text that Umuofian society places a great premium on the authority and wisdom of the Oracle and its leaders. As to whether it is right to condemn Umuofia for Ikemefuna's death, we must ask ourselves whether we agree with the Umuofian concept of justice.
In the beginning of the story, a woman from Umuofia was murdered by someone from the Mbaino clan. In response, Umuofia offered two alternatives to Mbaino: either both tribes went to war or a Mbaino young man and virgin girl were given to Umuofia as compensation for the woman's death. Mbaino, anxious to avoid war with the powerful Umuofia, had agreed to the second alternative. The text tells us that this was the "normal course of action," which leads us to conclude that Umuofian society thrived on strict interpretations of entrenched Igbo ethics.
To change the culture, dissent must be allowed. If we disagree with the concept of justice in Umuofia, we may well condemn all of Umuofia for Ikemefuna's death. If, however, we see some value in Umuofian justice, we may interpret Ikemefuna's death as the necessary sacrifice in response to an Umuofian woman's senseless murder. Regardless of our choice, it is plain from the text that there are those in Umuofia who have begun to question the traditions of the clan.