Well, my dear friend, I would suggest that someone hardens their own hearts when they refuse to honor and obey God, when they refuse His kind outreaches of mercy and grace to them. If this persists over time, God may at length allow the ungodly to go their own way. God will at length give them over to their own passions and lusts as we see in Romans 1:19-24. In other words, God will not always strive with men in an effort to draw them to receive His love and blessings (Genesis 6:3-5).

Now, as an example of what I am saying, let us take the verse you refer to in Exodus 9:12 where we read: “But the LORD hardened the heart of Pharaoh; and he did not heed them, just as the LORD had spoken to Moses.” So, does it seem that God hardened the heart of a just and righteous man in this verse? It seems to me that Pharoah had already made up his own mind about the children of Israel, even though Moses had clearly shown Pharoah many great signs from God that letting Israel go was the will of God. But was not Pharoah’s heart accountable in all of this? In Exodus 8:32 we read: “But Pharaoh hardened his heart at this time also; neither would he let the people go.” Pharoah had no intention of ever letting Israel go due to his own pride and his lack of belief in or fear of the living God.  Pharoah’s heart had already been revealed in Exodus 5:2 where we read, “And Pharaoh said, Who is the LORD, that I should obey his voice to let Israel go? I know not the LORD, neither will I let Israel go.”

But now you ask why God would still hold someone accountable if God should allow a hardened heart to sink further into hardness, which I believe was the case for Pharoah. My dear friend, mercy and grace are abundantly available to all who desire to love and obey God. This was true in OT times (Psalms 32:5), and it is still true today. Just look at God’s love in sending His only begotten Son into the world to be our sin offering (2 Corinthians 5:21; 1 John 4:10) so that all who believe on Christ, though we are all sinners, might be saved (John 3:16-17; Ephesians 2:8-9; Romans 10:9; John 10:10, and many other portions). God is a loving God who reached out to His chosen people and pleaded with them to come back to Himself (see Jeremiah 31:3 and many other OT Bible books of the major and minor prophets). But when people have spurned the merciful hand of God over time, He may no longer strive with them to come to Him for mercy and will allow them, by His own sovereign will, and without further warnings to be preserved for the judgment that is due them for their own sins. I believe we see an example of how God is faithful to preserve the godly from temptation, while allowing the ungodly to go their own course without any restraint in 2 Peter 2:9-10. But as we see in 2 Peter 3:9, “The Lord is not slack concerning His promise, as some count slackness, but is longsuffering toward us, not willing that any should perish but that all should come to repentance.”

So, we do have a loving and merciful God, and God has no pleasure in having to judge man. But God is just and righteous, so He must judge sin. All are sinners (Romans 3:23), but God is gracious and full of mercy, and this is why He has given us a Savior that we might have forgiveness for our sins and new life in Christ Jesus. But judgment upon those who refuse His mercy and grace will be sure. My dear friend, the Lord is a righteous judge and He knows those who are His own, those who truly love and honor Him (2 Timothy 2:19; John 6:37). All His judgments are perfectly righteous.  He makes no mistakes. We can be sure of this by the testimony of God’s Word, the Bible. Psalms 145:17 is but one of many verses which bring this out. I believe that God’s Word sums up the answer to your question very well: “For the Scripture says to Pharaoh, “For this very purpose I have raised you up, that I may show My power in you, and that My name may be declared in all the earth.” Therefore, He has mercy on whom He wills, and whom He wills He hardens. You will say to me then, “Why does He still find fault? For who has resisted His will?” But indeed, O man, who are you to reply against God? Will the thing formed say to him who formed it, “Why have you made me like this?” (Romans 9: 17-20).  (SF)  (706.1)