Why did God rebuke Moses (peace be upon him) for killing the Coptic man, despite the Copts' oppression of the Muslims?

Moses (peace be upon him) felt remorse and was rebuked by his Lord for taking a life without permission, because an innocent person should not be held accountable for the crime of a transgressor, in accordance with His (the Almighty's) saying: "And no bearer of burdens will bear the burden of another." Moses repented, saying: "My Lord, indeed I have wronged myself, so forgive me," and Allah forgave him. His killing of the Copt was a mistake, as narrated by Ibn 'Umar from the Prophet (peace and blessings be upon him), mentioned in Sahih (definition) Muslim. Moses's age at that time was twelve years.