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)