Objection 1. It would seem that there can be no evil of fault in the angels. For there can be no evil except in things which are in potentiality, as is said by the Philosopher (Metaph. ix, text. 19), because the subject of privation is a being in potentiality. But the angels have not being in potentiality, since they are subsisting forms. Therefore there can be no evil in them.
Objection 2. Further, the angels are higher than the heavenly bodies. But philosophers say that there cannot be evil in the heavenly bodies. Therefore neither can there by in the angels.
Objection 3. Further, what is natural to a thing is always in it. But it is natural for the angels to be moved by the movement of love towards God. Therefore such love cannot be withdrawn from them. But in loving God they do not sin. Consequently the angels cannot sin.
Objection 4. Further, desire is only of what is good or apparently good. Now for the angels there can be no apparent good which is not a true good; because in them either there can be no error at all, or at least not before guilt. Therefore the angels can desire only what it truly good. But no one sins by desiring what is truly good. Consequently the angel does not sin by desire.
On the contrary, It is said (Job 4:18): "In His angels He found wickedness."
I answer that, An angel or any other rational creature considered in his own nature, can sin; and to whatever creature it belongs not to sin, such creature has it as a gift of grace, and not from the condition of nature. The reason of this is, because sinning is nothing else than a deviation from that rectitude which an act ought to have; whether we speak of sin in nature, art, or morals. That act alone, the rule of which is the very virtue of the agent, can never fall short of rectitude. Were the craftsman's hand the rule itself engraving, he could not engrave the wood otherwise than rightly; but if the rightness of engraving be judged by another rule, then the engraving may be right or faulty. Now the Divine will is the sole rule of God's act, because it is not referred to any higher end. But every created will has rectitude of act so far only as it is regulated according to the Divine will, to which the last end is to be referred: as every desire of a subordinate ought to be regulated by the will of his superior; for instance, the soldier's will, according to the will of his commanding officer. Thus only in the Divine will can there be no sin; whereas there can be sin in the will of every creature; considering the condition of its nature.
Reply to Objection 1. In the angels there is no potentiality to natural existence. Yet there is potentiality in their intellective part, as regards their being inclined to this or the other object. In this respect there can be evil in them.
Reply to Objection 2. The heavenly bodies have none but a natural operation. Therefore as there can be no evil of corruption in their nature; so neither can there be evil of disorder in their natural action. But besides their natural action there is the action of free-will in the angels, by reason of which evil may be in them.
Reply to Objection 3. It is natural for the angel to turn to God by the movement of love, according as God is the principle of his natural being. But for him to turn to God as the object of supernatural beatitude, comes of infused love, from which he could be turned away by sinning.
Reply to Objection 4. Mortal sin occurs in two ways in the act of free-will.
First, when something evil is chosen; as man sins by choosing adultery, which is evil of itself. Such sin always comes of ignorance or error; otherwise what is evil would never be chosen as good. The adulterer errs in the particular, choosing this delight of an inordinate act as something good to be performed now, from the inclination of passion or of habit; even though he does not err in his universal judgment, but retains a right opinion in this respect. In this way there can be no sin in the angel; because there are no passions in the angels to fetter reason or intellect, as is manifest from what has been said above (Question 59, Article 4); nor, again, could any habit inclining to sin precede their first sin. In another way sin comes of free-will by choosing something good in itself, but not according to proper measure or rule; so that the defect which induces sin is only on the part of the choice which is not properly regulated, but not on the part of the thing chosen; as if one were to pray, without heeding the order established by the Church. Such a sin does not presuppose ignorance, but merely absence of consideration of the things which ought to be considered. In this way the angel sinned, by seeking his own good, from his own free-will, insubordinately to the rule of the Divine will.
Source: Summa Theologica (New Advent)