I must first apologize for the fairly technical post, but I just find this so interesting. It is Aquinas’ take on the sin of Lucifer, explained as best I could in simple terms:
It has been said by various authorities that Lucifer sinned by seeking something good, and perhaps this position bears repetition. The devil’s sin is to seek unauthorized equality with God. However, it cannot be said that he seek divine equality absolutely because this would be contradictory to God’s essence on God’s part (nothing can be equal to God for he is ipsum esse subsistens [subsistent being itself]). Also, it cannot be possible that there should be two of this kind, mainly that participating esse (existence) cannot be equal to which is essentially the esse itself.
Further, we cannot blame the structure of the devil for his sin for it is natural to intelligence, even separated intelligence to understand its own substance. Therefore, naturally it knows that its being is participated being and not subsistent being itself. In Lucifer’s case, since there is a certain natural knowledge in him that has not yet been corrupted, he should have realized that his esse is participating esse.
Moreover, the will always desires something good either for itself or for another. But it cannot be said that Lucifer have sinned because he wanted divine equality for another, (for he could without sin want the Son to be equal to the Father) but he sought equality for himself. Further, if another angel were to be created and given authority which necessitates him to come closer to divine authority, Lucifer would not have cared what would happen to him. From this, it is apparent that the angel did not seek to change his identity. If, however, he were equal to God, even if this were possible, he would not be himself because his species would change if he was raised to a higher rank in nature. Likewise, he could not desire what was not absolutely subject to God. Then because this is impossible, neither could it occur to him as possible for he himself would cease to exist.
All angels, however, are designed so that whatever is of their perfection they would immediately hold it from the principle of their creation. Nevertheless they were in potentiality to supernatural good, which through the grace of God they were able to pursue. Therefore the result is that the devil’s sin would not be in anything pertaining to the natural order, but to something supernatural. So that Lucifer’s sin is that he did not raise himself to God as if desiring along with the holy angels the final perfection from God’s grace, but he willed to pursue it through the power of his own nature. And because to have final happiness through the power of his own nature, and not from the grace of God, it is clear how much for this reason the devil sought equality with God and how much for this reason also he sought not to be subjected to God so that evidently he would not need his grace beyond the power of his own nature. And this agrees with the premise that the fallen angel sinned, not by desiring something evil, rather something good, namely final happiness, without conferring God’s grace.