Objection 1. It would seem that hatred is a capital sin. For hatred is directly opposed to charity. Now charity is the foremost among the virtues, and the mother of all others. Therefore hatred is the chief of the capital sins, and the origin of all others.
Objection 2. Further, sins arise in us on account of the inclinations of our passions, according to Romans 7:5: "The passions of sins... did work in our members to bring forth fruit unto death." Now all other passions of the soul seem to arise from love and hatred, as was shown above (I-II, 25, 1,2). Therefore hatred should be reckoned one of the capital sins.
Objection 3. Further, vice is a moral evil. Now hatred regards evil more than any other passion does. Therefore it seems that hatred should be reckoned a capital sin.
On the contrary, Gregory (Moral. xxxi) does not reckon hatred among the seven capital sins.
I answer that, As stated above (I-II, 84, 3,4), a capital vice is one from which other vices arise most frequently. Now vice is contrary to man's nature, in as much as he is a rational animal: and when a thing acts contrary to its nature, that which is natural to it is corrupted little by little. Consequently it must first of all fail in that which is less in accordance with its nature, and last of all in that which is most in accordance with its nature, since what is first in construction is last in destruction. Now that which, first and foremost, is most natural to man, is the love of what is good, and especially love of the Divine good, and of his neighbor's good. Wherefore hatred, which is opposed to this love, is not the first but the last thing in the downfall of virtue resulting from vice: and therefore it is not a capital vice.
Reply to Objection 1. As stated in Phys. vii, text. 18, "the virtue of a thing consists in its being well disposed in accordance with its nature." Hence what is first and foremost in the virtues must be first and foremost in the natural order. Hence charity is reckoned the foremost of the virtues, and for the same reason hatred cannot be first among the vices, as stated above.
Reply to Objection 2. Hatred of the evil that is contrary to one's natural good, is the first of the soul's passions, even as love of one's natural good is. But hatred of one's connatural good cannot be first, but is something last, because such like hatred is a proof of an already corrupted nature, even as love of an extraneous good.
Reply to Objection 3. Evil is twofold. One is a true evil, for the reason that it is incompatible with one's natural good, and the hatred of such an evil may have priority over the other passions. There is, however, another which is not a true, but an apparent evil, which, namely, is a true and connatural good, and yet is reckoned evil on account of the corruption of nature: and the hatred of such an evil must needs come last. This hatred is vicious, but the former is not.
Source: Summa Theologica (New Advent)