He made a bad judgment to seek his ultimate good in his own nature.
God is supernatural, meaning above our nature, even above the nature of the angels. By our nature, we can not desire God - that requires grace. Likewise, the angels cannot naturally desire the supernatural end God destined them for. But they were given grace to make this decision, and they had to make an act of love and giving of themselves. This act would result in the reward of glory with God. (cf. Summa “TREATISE ON THE ANGELS” catholicprimer.org/summa/FP.html#TOC03)
But there would be no merit in it if it were forced. So God allows free will to reject grace. But rejecting grace is a demerit. But God provides all that is needed, just for the asking. So even God’s help has to be rejected. So if free will chooses to rely on itself in order to glory in itself, then it will naturally reject grace and any reward from God that requires gratitude to God. “I can do it myself”. “I don’t need you”. We see such behavior even in little children at times.
This is also why is stands to reason that the devil was the highest angel by nature. No one, not even the blessed in heaven, fully comprehends God (De fide, Ott p.23). The endowments given to the angels are much greater than ours. If it is easy for us to rejoice in our good fortune in such a way that we want to control it and do what we want with it, how much more so the highest angel, who influences the lower angels?
We must not glory in ourselves or our possessions; we must always be grateful to God from Whom all good things come. Taking pride in it as if it is now all we need is the loss of the fear of God, of recognizing our dependence upon Him, and the cutting off of ourselves from Him - leading us to cling by reflex to ourselves in a deadly “vice” grip, and we fall like a brick.
God’s justice is swift. Let us not reject His goodness, let us not remain in ignorance of our dependence upon Him, and how much we need to worship Him in order to appreciate Him as the source of our life and our love!
Yes, God is a great King, and has created us in our nature without consulting us! He has arrived and conquered our nothingness, giving us a will and thereby “forcing” us to make a decision one way or the other regarding His Mercy: to give Him our service in love and praise and gratitude in dependence upon His Mercy towards our nothingness, or to hate and curse and wretch independently of Him while under His Justice for not doing mercy. For apart from Him is no good.
Christ Jesus, Victor! Christ Jesus, Ruler! Christ Jesus, Lord and Redeemer!