Why does Okonkwo participate in the slaughter in spite of the elder's advice?

He feels that he has a duty to the gods and the Oracle to participate. He delivers the killing blow because when Ikemefuna cries out to him, he inwardly feels compassion, and worries that he might do something that would cause him to be seen as unmanly, like attempting to save the child. Okonkwo overcompensates by killing Ikemefuna himself, and ending the personal trial.