Objection 1. It would seem that evil is not the object and cause of hatred. For everything that exists, as such, is good. If therefore evil be the object of hatred, it follows that nothing but the lack of something can be the object of hatred: which is clearly untrue.
Objection 2. Further, hatred of evil is praise-worthy; hence (2 Maccabees 3:1) some are praised for that "the laws were very well kept, because of the godliness of Onias the high-priest, and the hatred of their souls [Douay: 'his soul'] had no evil." If, therefore, nothing but evil be the object of hatred, it would follow that all hatred is commendable: and this is clearly false.
Objection 3. Further, the same thing is not at the same time both good and evil. But the same thing is lovable and hateful to different subjects. Therefore hatred is not only of evil, but also of good.
On the contrary, Hatred is the opposite of love. But the object of love is good, as stated above (26, 1; 27, 1). Therefore the object of hatred is evil.
I answer that, Since the natural appetite is the result of apprehension (though this apprehension is not in the same subject as the natural appetite), it seems that what applies to the inclination of the natural appetite, applies also to the animal appetite, which does result from an apprehension in the same subject, as stated above (Question 26, Article 1). Now, with regard to the natural appetite, it is evident, that just as each thing is naturally attuned and adapted to that which is suitable to it, wherein consists natural love; so has it a natural dissonance from that which opposes and destroys it; and this is natural hatred. So, therefore, in the animal appetite, or in the intellectual appetite, love is a certain harmony of the appetite with that which is apprehended as suitable; while hatred is dissonance of the appetite from that which is apprehended as repugnant and hurtful. Now, just as whatever is suitable, as such, bears the aspect of good; so whatever is repugnant, as such, bears the aspect of evil. And therefore, just as good is the object of love, so evil is the object of hatred.
Reply to Objection 1. Being, as such, has not the aspect of repugnance but only of fittingness; because being is common to all things. But being, inasmuch as it is this determinate being, has an aspect of repugnance to some determinate being. And in this way, one being is hateful to another, and is evil; though not in itself, but by comparison with something else.
Reply to Objection 2. Just as a thing may be apprehended as good, when it is not truly good; so a thing may be apprehended as evil, whereas it is not truly evil. Hence it happens sometimes that neither hatred of evil nor love of good is good.
Reply to Objection 3. To different things the same thing may be lovable or hateful: in respect of the natural appetite, owing to one and the same thing being naturally suitable to one thing, and naturally unsuitable to another: thus heat is becoming to fire and unbecoming to water: and in respect of the animal appetite, owing to one and the same thing being apprehended by one as good, by another as bad.
Source: Summa Theologica (New Advent)