v 15 Paul quoted Ex 33:19 and who shows God’s mercy and compassion are extended according to God’s will and not mans. The reference deals with Israel’s idolatry while Moshe receiving the law.
Paul then quoted Ex 9:16 using Pharaoh as an illustration. Moshe a hebrew, Pharaoh a gentile, both committed murder. Both saw God’s wonders. Moshe saved, Pharaoh lost. One ruler, one a slave. This isn’t a matter of righteousness but the soverignity and will of God.
“Hardening” of Pharaoh (Rom 9:18). This process is mentioned at least 15 times in Ex 7-14. Sometimes Pharao hardened his heart (Ex 8:15, 19, 32), other times God hardened the heart (Ex 9:12; 10:1, 20, 27).
By declaring His word and revealing His power, God gave Pharaoh opportunity to repent. Pharaoh resisted God and hardened his heart.
The same sun that melts the ice, can harden the clay.