According to my story of the second creation, in particular of the sanctification of the angels, it was pride that caused some of the angelic creatures to refuse the grace given to them by the Holy Spirit. These angels would not stoop to serving humans. A war ensued, as a result of which the demons were cast out of their natural heavenly home and entrenched in this world as its rulers and enemies of mankind.
They aim at our destruction which would defy the Holy Spirit’s vision of a unified creation. Upon victory over us, the demons further intend to recapture heaven from the good angels and live there in their natural state forever.
However, pride would only ensure that the demons felt contempt for humans. They would still try to destroy us, but they would not waste any emotions on us. They would indeed consider us filth, cockroaches, meddlesome pests, but just as the exterminator does not hate the vermin even as he kills them by the thousands, neither would the demons hate us. They would work rather dispassionately in order to attain a definite end.
Traditionally, however, the demons are motivated by another emotion, viz., envy. The demons realize that the battle is far from being determined. They know they can (and hopefully will) lose and be imprisoned in the demonic hell. If we win, we will obtain 3 things.
First, the kingdom. We will become children of God and fully part of the divine family. No mystery or pleasure of God will be closed to us. We’ll inherit both paradise and heaven, from the latter of which the demons were explicitly exiled.
Second, the power. We will have defeated an enemy that is naturally superior to us, in fact led by the most spectacular creature out there, Lucifer, in a no-holds-barred merciless combat. In so doing, we’ll have proved ourselves greater than they. We will be raised above them in the exaltation of might, triumph, and dominion.
Third, the glory. While the demons hope merely to keep their natural happiness, humans are promised a far greater destiny: the glory of the saints. Our grace here is the beginning of the glory in the hereafter. It is the clarity of perfection, honor, and charity of the unashamed soul and participation in the inner life of God.
Understanding all this, the demons envy us for the favor shown to us by God. But envy is a species of hatred. The demons consider our good to “conduce to the lessening of their own good name or excellence,” as St. Thomas puts it. They sorrow over the fact that the alleged pests could surpass them. And that is why they hate us and are far from dispassionate in their feelings. But this only aggravates their sin.