Does the Bible explain why wouldn't God be merciful to the Devil?

God's answer to Job offers insight. God descibed many creatures, in each case commenting on some aspect of their creation, abilities and character. Each creature is exactly as He intended it, and Job is no exception. The book asserts God's sovereignty. Just as God confidently predicted Job's unwavering loyalty in the midst of tragedy prior to Satan's tormenting of the man, God also describes the character of Leviathan.

Nothing on earth is its equal—
    a creature without fear.
34 It looks down on all that are haughty;
    it is king over all that are proud.” (Job 41:33-34)

Down through history, most theologians agree that Leviathan as here depicted is the same creature called the "beast from the sea" in Revelation 13. That means that Leviathan - even if a real creature - in metaphor stands for Satan. (It has also been called an evil kingdom, evil world system, the embodiment of impersonal evil, the chaos of nature and impersonal natural disasters, etc. It could be all those things at the same time.)

The important words in verse 33 are "a creature without fear". Satan does not fear God. That is his inalterable nature. Since the fear of the Lord is the beginning of Wisdom, Satan can never become wise, can never change, never repent, never be saved.

God is love. God is forgiving. Does that obligate God to save Satan to prove He is infinite love?

Who has a claim against me that I must pay?
    Everything under heaven belongs to me. (Job 41:11)

No! God is not obligated to save Satan, or anyone. God is sovereign and free to do as he chooses.

It is interesting to read the rest of Job 41. God poses a series of rhetorical questions, asking which of many beneficial actions Leviathan will take on behalf of one who asks him for help. Leviathan - Satan - will do none of those things for us.

Jesus did them all. Jesus is the savior by definition. Leviathan is the anti-savior, by definition. Jesus will never change; he is savior for all time, with no turning away. Leviathan will never change, either, and Revelation proves it.

“I will not fail to speak of Leviathan’s limbs,
    its strength and its graceful form." (Job 41:12)

God is not just Sovereign, He delights in His creatures. He is happy that each one turned out exactly as he desired. That includes Leviathan. God likes Leviathan just the way it is, and He likes His plan to bind and imprison Leviathan in Hell for eternity. God won't change this because His plan pleases Him and He is carrying it out as advertised.

God may very well be merciful to Satan and his demons.

You haven't specified a denomination from which this question should be viewed. But there are Christian denominations that do not believe in the non-biblical concept of the unsaved being tortured forever.

The book of Revelation indicates that following the millennial paradise of the kingdom of God, the vast majority of mankind will be resurrected (the second general resurrection) and will be given their chance of salvation.

It is only those few that still refuse to accept God's way, including those in the third resurrection (people in this present age that accepted God's spirit and then later chose to reject it) that will be destroyed, burned to ashes. Real hellfire is permanent destruction, not eternal torment.

God is love, not a psychopathic vindictive god, getting pleasure from eternally torturing those that disobeyed him. As we would with a sick or vicious dog, God will mercifully and permanently end their lives.

So it will likely be with Satan and his demons.

In Revelation 20:10, the expression "for ever and ever" is the common translation of "eis tous aionas ton aionon". But it means "unto the ages of the ages", which could simply mean until the end of the final age, or end of the world, not literally for ever and ever. Satan and his demons may very well be permanently destroyed according to God's mercy and love.

God is love It would be cruel and mean to not forgive Satan my guess is Satan will be forgiven but nobody knows when god is love and forgiveness so he will forgive Satan and destroy him this is my guess

Evidence of Satan being decreed to Hell is found in Mathew 25:41

Mathew 25: 41-46

41 Then he will say also to those on the left hand, ‘Depart from me, you cursed, into the eternal fire which is prepared for the devil and his angels; 42 for I was hungry, and you didn’t give me food to eat; I was thirsty, and you gave me no drink; 43 I was a stranger, and you didn’t take me in; naked, and you didn’t clothe me; sick, and in prison, and you didn’t visit me.’

44 “Then they will also answer, saying, ‘Lord, when did we see you hungry, or thirsty, or a stranger, or naked, or sick, or in prison, and didn’t help you?’

45 “Then he will answer them, saying, ‘Most certainly I tell you, because you didn’t do it to one of the least of these, you didn’t do it to me.’ 46 These will go away into eternal punishment, but the righteous into eternal life.”

Explanation as to why in Isaiah 14:12-15

"How you are fallen from heaven, O Lucifer, son of the morning! How you are cut down to the ground, You who weakened the nations! For you have said in your heart: 'I will ascend into heaven, I will exalt my throne above the stars of God; I will also sit on the mount of the congregation On the farthest sides of the north; I will ascend above the heights of the clouds, I will be like the Most High.' Yet you shall be brought down to Sheol, To the lowest depths of the Pit.

Satan is guilty of blasphemy, an unforgivable sin.

Ezekiel 28:14-15 14 Thou art the anointed cherub that covereth, and I have set thee so; thou wast upon the holy mountain of God; thou hast walked up and down in the midst of the stones of fire. 15 Thou wast perfect in thy ways from the day that thou wast created, till iniquity was found in thee.

Isaiah 14 13 You said in your heart, “I will ascend to the heavens; I will raise my throne above the stars of God; I will sit enthroned on the mount of assembly, on the utmost heights of Mount Zaphon. 14 I will ascend above the tops of the clouds; I will make myself like the Most High.”

Does the Bible explain why wouldn't God be merciful to the Devil?

Satan and all the fallen angels (demons) are completely consumed with evil and chose to do evil with all their intellect and sin against God, thus repentance is no longer possible for them. They do not desire God’s mercy. In fact, they hate all that is good in man as well as God.

Being banished to hell is in itself a form of mercy towards the Devil. Satan hates God, the holy Angels and the Saints so much that to be told to remain in heaven would be more intolerant and cruel than all his torments in hell. The evil ones see things differently than us mere mortals.

Evil came into “being” because of the choice of one angel who chose to sin against God by not wanting to be obedient to His Authority as Lord and Creator. That angel, a high ranking cherub angel, was named Lucifer. And once he chose to rebel against God, he took about a third of the angels with him in rebellion (Revelation 12:4). Lucifer thus became the Devil and Satan, which means accuser and adversary of God, and he and his fallen angels became devils/demons. But Saint Michael the Archangel, along with Saint Gabriel, Saint Raphael, Uriel, Raguel, Zerachiel, and Remiel, led the rest of the loyal angels to God against Satan and cast Satan and his demons out of Heaven and into Hell. (Revelation 12:7-9, 2 Peter 2:4, Jude 6).

But why did Lucifer sin against God and dare to say “Non serviam!” (“I will not serve!”)? Because of pride (Isaiah 14:12-14, Ezekiel 28:17, Proverbs 16:18) and envy (Wisdom 2:24 – But through the devil's envy death entered the world, and those who belong to his party experience it). Lucifer was prideful of his high status, power, wisdom, and beauty, but what he had was not enough for he wanted to be adored and worship like God. Therefore, he did not want to be second to God in honor and so he chose to not serve his Master. Somehow, this rebellious idea of not serving God then appealed to the other angels who sided with Satan.

According to Canon 1 of the VI Lateran Council in 1215, the fallen angels were good. Does any goodness still exist within these demons and can they repent since God is merciful to sinful humans who repent of their evil? According to the teaching of the Catholic Church, no. There is not good left within Satan and his angels and they are completely consumed by evil. As a result of this and because they chose to sin against God with their full intellect and will. As Paragraph 393 of the Catechism of the Catholic Church states:

It is the irrevocable character of their choice, and not a defect in the infinite divine mercy, that makes the angels' sin unforgivable. "There is no repentance for the angels after their fall, just as there is no repentance for men after death."

In the same manner, it is thus argued that it is also the irrevocable character of the choice that the other angels who remained loyal to God made and thus they cannot give into sin because their wills are now so attuned to God’s Will. Thus, according to this doctrine of the Catholic Church, it is now impossible for a second rebellion of angels to ever take place. - The Catholic Teachings On The Angels - Part 2: The Fall Of Satan

Before the death of a human being, man is able to repent of past sins, but not after death.

Essentially, Catholics believe that after death, the soul is inclined towards good (God) or evil (pride or opposition to God) and there is no changing sides. The story of Lazarus points this out quite well.

What is true for man is true for Satan and all the demons in hell. Like man, the Devil had his chance.

Satan does not desire nor deserve God’s mercy. He desires the fall of every single person on earth. He tempted Jesus in the desert and persecuted God’s servant Job.

It is not that God is not merciful towards Satan and his satellites as much as they refuse to have mercy shown them. Satan hates God, the Holy Angels, and all mankind and desires all men to join him in hell, just as all the damned do.

Article(s) of interest can be perused below:

A letter from Beyond (from Hell)