In the beginning, before the creation of heaven and earth, God made the angels; free intelligences and free wills; out of His love He made them, that they might be eternally happy. And that their happiness might be complete, He gave them the perfection of a created nature; that is, He gave them freedom.
But happiness is only attainable by the free will agreeing in its freedom to accord with the will of God. Some of the angels by an act of free will obeyed the will of God, and in such obedience found perfect happiness; other angels by an act of free will rebelled against the will of God, and in such disobedience found misery.
Such is the catholic theory of the fall of the angels.