How did Moses pray to his Lord and ask for forgiveness for killing the Israelite before he knew of the existence of God and received revelation?

Moses, peace be upon him, was a monotheist before his prophethood, for he was infallible from polytheism, like all other prophets. Traces of Joseph's call, peace be upon him, remained in the hearts of the Children of Israel. Pharaoh and his people were aware of the existence of Allah Almighty, but they denied it. Messengers address people with what is known to them.