Objection 1. It would seem that we ought not to love sinners out of charity. For it is written (Psalm 118:113): "I have hated the unjust." But David had perfect charity. Therefore sinners should be hated rather than loved, out of charity.
Objection 2. Further, "love is proved by deeds" as Gregory says in a homily for Pentecost (In Evang. xxx). But good men do no works of the unjust: on the contrary, they do such as would appear to be works of hate, according to Psalm 100:8: "In the morning I put to death all the wicked of the land": and God commanded (Exodus 22:18): "Wizards thou shalt not suffer to live." Therefore sinners should not be loved out of charity.
Objection 3. Further, it is part of friendship that one should desire and wish good things for one's friends. Now the saints, out of charity, desire evil things for the wicked, according to Psalm 9:18: "May the wicked be turned into hell [Douay and A.V.: 'The wicked shall be,' etc. See Reply to this Objection.]." Therefore sinners should not be loved out of charity.
Objection 4. Further, it is proper to friends to rejoice in, and will the same things. Now charity does not make us will what sinners will, nor to rejoice in what gives them joy, but rather the contrary. Therefore sinners should not be loved out of charity.
Objection 5. Further, it is proper to friends to associate together, according to Ethic. viii. But we ought not to associate with sinners, according to 2 Corinthians 6:17: "Go ye out from among them." Therefore we should not love sinners out of charity.
On the contrary, Augustine says (De Doctr. Christ. i, 30) that "when it is said: 'Thou shalt love thy neighbor,' it is evident that we ought to look upon every man as our neighbor." Now sinners do not cease to be men, for sin does not destroy nature. Therefore we ought to love sinners out of charity.
I answer that, Two things may be considered in the sinner: his nature and his guilt. According to his nature, which he has from God, he has a capacity for happiness, on the fellowship of which charity is based, as stated above (3; 23, 1,5), wherefore we ought to love sinners, out of charity, in respect of their nature.
On the other hand their guilt is opposed to God, and is an obstacle to happiness. Wherefore, in respect of their guilt whereby they are opposed to God, all sinners are to be hated, even one's father or mother or kindred, according to Luke 12:26. For it is our duty to hate, in the sinner, his being a sinner, and to love in him, his being a man capable of bliss; and this is to love him truly, out of charity, for God's sake.
Reply to Objection 1. The prophet hated the unjust, as such, and the object of his hate was their injustice, which was their evil. Such hatred is perfect, of which he himself says (Psalm 138:22): "I have hated them with a perfect hatred." Now hatred of a person's evil is equivalent to love of his good. Hence also this perfect hatred belongs to charity.
Reply to Objection 2. As the Philosopher observes (Ethic. ix, 3), when our friends fall into sin, we ought not to deny them the amenities of friendship, so long as there is hope of their mending their ways, and we ought to help them more readily to regain virtue than to recover money, had they lost it, for as much as virtue is more akin than money to friendship. When, however, they fall into very great wickedness, and become incurable, we ought no longer to show them friendliness. It is for this reason that both Divine and human laws command such like sinners to be put to death, because there is greater likelihood of their harming others than of their mending their ways. Nevertheless the judge puts this into effect, not out of hatred for the sinners, but out of the love of charity, by reason of which he prefers the public good to the life of the individual. Moreover the death inflicted by the judge profits the sinner, if he be converted, unto the expiation of his crime; and, if he be not converted, it profits so as to put an end to the sin, because the sinner is thus deprived of the power to sin any more.
Reply to Objection 3. Such like imprecations which we come across in Holy Writ, may be understood in three ways: first, by way of prediction, not by way of wish, so that the sense is: "May the wicked be," that is, "The wicked shall be, turned into hell." Secondly, by way of wish, yet so that the desire of the wisher is not referred to the man's punishment, but to the justice of the punisher, according to Psalm 57:11: "The just shall rejoice when he shall see the revenge," since, according to Wisdom 1:13, not even God "hath pleasure in the destruction of the wicked [Vulgate: 'living']" when He punishes them, but He rejoices in His justice, according to Psalm 10:8: "The Lord is just and hath loved justice." Thirdly, so that this desire is referred to the removal of the sin, and not to the punishment itself, to the effect, namely, that the sin be destroyed, but that the man may live.
Reply to Objection 4. We love sinners out of charity, not so as to will what they will, or to rejoice in what gives them joy, but so as to make them will what we will, and rejoice in what rejoices us. Hence it is written (Jeremiah 15:19): "They shall be turned to thee, and thou shalt not to be turned to them."
Reply to Objection 5. The weak should avoid associating with sinners, on account of the danger in which they stand of being perverted by them. But it is commendable for the perfect, of whose perversion there is no fear, to associate with sinners that they may convert them. For thus did Our Lord eat and drink with sinners as related by Matthew 9:11-13. Yet all should avoid the society of sinners, as regards fellowship in sin; in this sense it is written (2 Corinthians 6:17): "Go out from among them... and touch not the unclean thing," i.e. by consenting to sin.
Source: Summa Theologica (New Advent)