Objection 1. It would seem that we ought to love the demons out of charity. For the angels are our neighbors by reason of their fellowship with us in a rational mind. But the demons also share in our fellowship thus, since natural gifts, such as life and understanding, remain in them unimpaired, as Dionysius states (Div. Nom. iv). Therefore we ought to love the demons out of charity.
Objection 2. Further, the demons differ from the blessed angels in the matter of sin, even as sinners from just men. Now the just man loves the sinner out of charity. Therefore he ought to love the demons also out of charity.
Objection 3. Further, we ought, out of charity, to love, as being our neighbors, those from whom we receive favors, as appears from the passage of Augustine quoted above (Article 9). Now the demons are useful to us in many things, for "by tempting us they work crowns for us," as Augustine says (De Civ. Dei xi, 17). Therefore we ought to love the demons out of charity.
On the contrary, It is written (Isaiah 28:18): "Your league with death shall be abolished, and your covenant with hell shall not stand." Now the perfection of a peace and covenant is through charity. Therefore we ought not to have charity for the demons who live in hell and compass death.
I answer that, As stated above (Article 6), in the sinner, we are bound, out of charity, to love his nature, but to hate his sin. But the name of demon is given to designate a nature deformed by sin, wherefore demons should not be loved out of charity. Without however laying stress on the word, the question as to whether the spirits called demons ought to be loved out of charity, must be answered in accordance with the statement made above (A2,3), that a thing may be loved out of charity in two ways. First, a thing may be loved as the person who is the object of friendship, and thus we cannot have the friendship of charity towards the demons. For it is an essential part of friendship that one should be a well-wisher towards one's friend; and it is impossible for us, out of charity, to desire the good of everlasting life, to which charity is referred, for those spirits whom God has condemned eternally, since this would be in opposition to our charity towards God whereby we approve of His justice.
Secondly, we love a thing as being that which we desire to be enduring as another's good. On this way we love irrational creatures out of charity, in as much as we wish them to endure, to give glory to God and be useful to man, as stated above (Article 3): and in this way too we can love the nature of the demons even out of charity, in as much as we desire those spirits to endure, as to their natural gifts, unto God's glory.
Reply to Objection 1. The possession of everlasting happiness is not impossible for the angelic mind as it is for the mind of a demon; consequently the friendship of charity which is based on the fellowship of everlasting life, rather than on the fellowship of nature, is possible towards the angels, but not towards the demons.
Reply to Objection 2. In this life, men who are in sin retain the possibility of obtaining everlasting happiness: not so those who are lost in hell, who, in this respect, are in the same case as the demons.
Reply to Objection 3. That the demons are useful to us is due not to their intention but to the ordering of Divine providence; hence this leads us to be friends, not with them, but with God, Who turns their perverse intention to our profit.
Source: Summa Theologica (New Advent)