If a third (or a large number) of angels rebelled, what was the justification?
We know not the exact moment the angels were created, but we know that they were created naturally good. Although, at the moment of their creation, they did not enjoy the Beatific Vision, for this is reserved for the Holy Angels who never sinned or rebelled against God.
The angels at the moment of their creation were in heaven that is to say the immediate atmosphere that surrounds the earth and the entire external physical universe. They were not in the intimate presence of God in Heaven.
St. Thomas Aquinas as well as others believe the sin of the fallen angels was one of pride and envy.
Sin can exist in a subject in two ways: first of all by actual guilt, and secondly by affection. As to guilt, all sins are in the demons; since by leading men to sin they incur the guilt of all sins. But as to affection only those sins can be in the demons which can belong to a spiritual nature. Now a spiritual nature cannot be affected by such pleasures as appertain to bodies, but only by such as are in keeping with spiritual things; because nothing is affected except with regard to something which is in some way suited to its nature. But there can be no sin when anyone is incited to good of the spiritual order; unless in such affection the rule of the superior be not kept. Such is precisely the sin of pride — not to be subject to a superior when subjection is due. Consequently the first sin of the angel can be none other than pride.
Yet, as a consequence, it was possible for envy also to be in them, since for the appetite to tend to the desire of something involves on its part resistance to anything contrary. Now the envious man repines over the good possessed by another, inasmuch as he deems his neighbor's good to be a hindrance to his own. But another's good could not be deemed a hindrance to the good coveted by the wicked angel, except inasmuch as he coveted a singular excellence, which would cease to be singular because of the excellence of some other. So, after the sin of pride, there followed the evil of envy in the sinning angel, whereby he grieved over man's good, and also over the Divine excellence, according as against the devil's will God makes use of man for the Divine glory. - Whether only the sin of pride and envy can exist in an angel?
Thus it seems that most probably Satan sinned immediately after his own creation and there was no interval of time in his decision.
There is a twofold opinion on this point. But the more probable one, which is also more in harmony with the teachings of the Saints, is that the devil sinned at once after the first instant of his creation. This must be maintained if it be held that he elicited an act of free-will in the first instant of his creation, and that he was created in grace; as we have said (I:62:3. For since the angels attain beatitude by one meritorious act, as was said above (I:62:5), if the devil, created in grace, merited in the first instant, he would at once have received beatitude after that first instant, if he had not placed an impediment by sinning.
If, however, it be contended that the angel was not created in grace, or that he could not elicit an act of free-will in the first instant, then there is nothing to prevent some interval being interposed between his creation and fall. - Whether there was any interval between the creation and the fall of the angel?
As to the exact sin of the fallen angels, it seems most probable that they desired to be as God. This can be seen in the very beginning in Genesis when the Deceiver lied to Eve in the Garden of Eden.
The serpent’s deception in the Garden had included a grain of truth. Satan told Eve, “God knows that when you eat from it your eyes will be opened, and you will be like God, knowing good and evil” (Genesis 3:5). What the serpent did not say was that knowing evil would damage Adam and Eve’s relationship with God. Half-truths can be as deceptive as full-blown lies. - How did the knowledge of good and evil make man like God (Genesis 3:22)?
Again this desire to be as God is echoed in the writings of Thomas Aquinas and the the sin of highest angel could be the cause of the angels to follow in kind.
The sin of the highest angel was the cause of the others sinning; not as compelling them, but as inducing them by a kind of exhortation. A token thereof appears in this, that all the demons are subjects of that highest one; as is evident from our Lord's words: "Go [Vulg. 'Depart from Me'], you cursed, into everlasting fire, which was prepared for the devil and his angels" (Matthew 25:41). For the order of Divine justice exacts that whosoever consents to another's evil suggestion, shall be subjected to him in his punishment; according to (2 Peter 2:19): "By whom a man is overcome, of the same also he is the slave." - Whether the sin of the highest angel was the cause of the others sinning?
Some theologians postulate that the pride that caused the sin of the fallen angels was due to the incarnation of Christ. They refused to serve beings of an inferior nature!
It is the non serviam of Satan that messed up everything at the demonic rebellion against God.
Although nothing definite can be known as to the precise nature of the probation of the angels and the manner in which many of them fell, many theologians have conjectured, with some show of probability, that the mystery of the Divine Incarnation was revealed to them, that they saw that a nature lower than their own was to be hypostatically united to the Person of God the Son, and that all the hierarchy of heaven must bow in adoration before the majesty of the Incarnate Word; and this, it is supposed, was the occasion of the pride of Lucifer (cf. Suarez, De Angelis, lib. VII, xiii). As might be expected, the advocates of this view seek support in certain passages of Scripture, notably in the words of the Psalmist as they are cited in the Epistle to the Hebrews: "And again, when he bringeth in the first-begotten into the world, he saith: And let all the angels of God adore Him" (Hebrews 1:6; Psalm 96:7). And if the twelfth chapter of the Apocalypse may be taken to refer, at least in a secondary sense, to the original fall of the angels, it may seem somewhat significant that it opens with the vision of the Woman and her Child. But this interpretation is by no means certain, for the text in Hebrews 1, may be referred to the second coming of Christ, and much the same may be said of the passage in the Apocalypse.
It would seem that this account of the trial of the angels is more in accordance with what is known as the Scotist doctrine on the motives of the Incarnation than with the Thomist view, that the Incarnation was occasioned by the sin of our first parents. For since the sin itself was committed at the instigation of Satan, it presupposes the fall of the angels. How, then, could Satan's probation consist in the fore-knowledge of that which would, ex hypothesi, only come to pass in the event of his fall? In the same way it would seem that the aforesaid theory is incompatible with another opinion held by some old theologians, to wit, that men were created to fill up the gaps in the ranks of the angels. For this again supposes that if no angels had sinned no men would have been made, and in consequence there would have been no union of the Divine Person with a nature lower than the angels. - Devil