Moses was a Jew, whereas Pharoah was a Gentile, but both of them were sinners. Both were murderers, and both witnessed God’s miracles. Yet, Moses was redeemed and Pharoah was not. God raised up Pharoah in order to reveal His own glory and power, and God had mercy on Moses in order to use him to deliver His people, Israel. Pharoah was a ruler, whereas Moses’ people were slaves under Pharoah. But Moses received God’s mercy and compassion because that was God’s will. The Lord’s work is sovereign, and He acts entirely according to His own will to accomplish His own purposes. The issue was not the presumed rights of either man but, rather, the sovereign will of God.
From: The MacArthur New Testament Commentary by John MacArthur; 33 volumes (Chicago: Moody Publishers, 1983-2015), 16:35 (the volume covering Romans 9-16, at Romans 9.14-18, published in 1994)