[Eternal Abiding in the Fire]

Answer (and Allah is the Giver of success):
 Pardoning the criminal is, according to reason, something good, and in general permissible according to its rulings. It is not obligatory upon the one who has a right to take his right in full; rather, he may take or leave (it), whether in presence or in absence. Thus it has been established in the judgments of intellects.
 However, there may arise, in the way of beneficence, an impediment that prevents judging that beneficence to be good, so that beneficence, because of that incidental impediment, becomes ugly (reprehensible). The attribution of ugliness to beneficence in such a case is only an incidental attribution.
If it is said: Goodness (al-ḥusn) and beneficence (al-iḥsān) are among the attributes whose essences do not change. So if an informant were to report and say, “Beneficence is ugly,” or “Goodness is ugly,” his report would be deemed false, and it would be like the statement of one who says, “The motionless is moving,” or “The triangle is a square,” or “The land is a sea.” Such a statement is invalid and unacceptable because it is information about a thing in terms of what is contrary to its very essence, or at odds with it; and what is known to the intellect is that the essence of a thing does not change.
It is said in response: Yes, beneficence does not change, as you have said, and it is impossible for beneficence to be ugly.
What we mean is that beneficence becomes ugly not insofar as it is beneficence, but because of an ugly adjunct that is joined to it and adheres to it, such that performing the act of beneficence entails the occurrence of that ugliness which inseparably accompanies it. Let us strike for that some examples:
First example: Speaking the truth is good, and it is inconceivable that truth, insofar as it is truth, should cease to be characterized by this attribute and become ugly. However, there may be an adjunct that joins to the speaking of truth whereby the truth becomes ugly, not because it is truth, but because of the supervening adjunct that has joined it and accompanies it. For example, suppose an oppressor is searching for an innocent believing man in order to kill him, or to torture him, or to take his wealth, and you know the man and his whereabouts. Then that oppressor asks you about the man and his location in order to kill him or … while he is able to seize him and kill him. In such a case, telling the truth to the oppressor would be ugly and a crime for which the speaker deserves Allah’s loathing and wrath. This is clear, for any rational person perceives its ugliness by his intellect alone. And there is no doubt that that the ugliness we find in that truthful statement does not arise from the truthfulness itself, but rather the ugliness is due to what has been joined to the telling of truth — namely, the occurrence of the believer’s killing, the taking of his wealth, and his torture.
Thus you see in this example what makes clear to you that something good can become ugly due to the occurrence of an incidental factor that stands in the way of that goodness.
Second example: If among the sultan’s subjects there is someone who tyrannizes the people through killing, plundering, and harming them, and then the sultan seizes him, knowing from his condition that he will not cease from that conduct, and knowing that pardoning him would only increase him in rebellion and aggression, then the pardon in such a case would be ugly — not because it is pardon, but because of what is joined to that pardon and inseparably linked to it of the occurrence of evil and harm upon the subjects. There is no doubt that all the subjects would be angered with the sultan when he pardons him and would censure him, and their tongues would be loosed in reviling and cursing him. This is not because he pardoned — for pardon is good, and the one who does it deserves praise and commendation — but because of what has been joined to his pardon of the occurrence of ugliness.
So if you, O questioner, have clearly understood what we have mentioned, and it has become evident to you what we have explained, then know that Allah’s pardon on the Day of Reckoning of those who died persisting in their disbelief, denial, and defiant disobedience is not good, because of what would accompany that pardon on that day and join to it of ugliness.
The clarification of this is that Allah the Exalted has threatened the criminals, the flagrantly disobedient, the disbelievers, and the hypocrites with severe threat and everlasting punishment in Hell — a confirmed threat — and He has repeated that many times in His Book.
And He accompanied that severe threat by describing Himself, Exalted is He, as One who does not fail in His promise, and as One with whom there is no changing of the word, and that His word is the truest of all words and His speech the most truthful of all speech. He has sworn to His servants many oaths in His Wise Book that He will punish the hypocrites, the disbelievers, the wicked, the wrongdoers, and the arrogant with everlasting, eternal torment.
So if we hypothesize and allow that Allah the Exalted will pardon those people or some of them, then there would be joined to this hypothesis and permissibility the accusation of Allah the Exalted of lying, of breaking His promise, and of His oaths and pledges in His Book not being truthful.
Thus, allowing for the possibility of pardon from Allah on the Day of Resurrection would entail allowing for the possibility of falsehood in the Qur’an — and there is no doubt that allowing for the possibility of falsehood in the Qur’an is disbelief and hypocrisy.
Furthermore, everything in the Qur’an is truth, "Falsehood cannot approach it from behind it; [It is] a revelation from a [Lord who is] Wise and Praiseworthy" [Fussilat:42], and that is a matter of consensus among the Muslims. Truth is the opposite of falsehood, and allowing for the possibility of pardon (in this sense) amounts to allowing that this verse is not true.

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.3