A person may commit a great sin or many sins causing the judgment rendered before the True Judge to be that the retribution [administered to] this transgressor for these sins which he willfully and consciously committed is that his Teshuvah will be held back. He will not be allowed the chance to repent from his wickedness so that he will die and be wiped out because of the sin he committed.
This is implied by the Holy One, blessed be He's statement [related] by Isaiah [6:10]: “Make the heart of this people fat [and make their ears heavy. Smear over their eyes, lest they see with their eyes... understand with their hearts, repent and be healed].”
Similarly, [II Chronicles 36:16] states “ They mocked the messengers of God, scorned His words, scoffed at His prophets until the anger of God mounted up against His people until there was no remedy.”
Implied [by these verses] is that they willingly sinned, multiplying their iniquity until it was obliged to hold back their Teshuvah, [which is referred to as] the "remedy."
For these reasons, it is written in the Torah [Exodus 14:4], "I will harden Pharaoh's heart." Since, he began to sin on his own initiative and caused hardships to the Israelites who dwelled in his land as [Exodus 1:10] states: "Come, let us deal wisely with them," judgment obligated that he be prevented from repenting so that he would suffer retribution. Therefore, The Holy One, blessed be He, hardened his heart.
Why did [God] send Moses to [Pharaoh], telling him: “Send [forth the people], repent”? The Holy One, blessed be He, had already told that he would not release [the people], as [Exodus 9:30] states: “I realize that you and your subjects [still do not fear God].”
[The reason is stated in Exodus 9:16:] “For this alone, I have preserved you… so that My name will be spoken about throughout the earth],” i.e., to make known to all the inhabitants of the world that when the Holy One, blessed be He, withholds repentance from a sinner, he cannot repent, but he will die in the wickedness that he initially committed willfully.
Similarly, Sichon was held liable for repentance to be withheld from him, because of the sins he committed, as [Deuteronomy 2:30] states: “God, your Lord, hardened his spirit and strengthened his heart.”
Also, the Canaanites held back from repenting, because of their abominable acts, so that they would wage war against Israel as [Joshua 11:20] states: "This was inspired by God, to harden their hearts so that they should come against Israel in battle in order to utterly destroy them."
Similarly, the Israelites during the era of Elijah committed many iniquities. Repentance was held back from those who committed these many sins, as [I Kings 18:37] states: "You have turned their heart backwards," i.e., held repentance back from them.
In conclusion, the Almighty did not decree that Pharaoh should harm the Israelites that Sichon should sin in his land, that the Canaanites should perform abominable acts, or that the Israelites should worship idols. They all sinned on their own initiative and they were obligated to have Teshuvah held back from them.