Objection 1. It would seem that a man can hate himself. For it is written (Psalm 10:6): "He that loveth iniquity, hateth his own soul." But many love iniquity. Therefore many hate themselves.
Objection 2. Further, him we hate, to whom we wish and work evil. But sometimes a man wishes and works evil to himself, e.g. a man who kills himself. Therefore some men hate themselves.
Objection 3. Further, Boethius says (De Consol. ii) that "avarice makes a man hateful"; whence we may conclude that everyone hates a miser. But some men are misers. Therefore they hate themselves.
On the contrary, The Apostle says (Ephesians 5:29) that "no man ever hated his own flesh."
I answer that, Properly speaking, it is impossible for a man to hate himself. For everything naturally desires good, nor can anyone desire anything for himself, save under the aspect of good: for "evil is outside the scope of the will," as Dionysius says (Div. Nom. iv). Now to love a man is to will good to him, as stated above (Question 26, Article 4). Consequently, a man must, of necessity, love himself; and it is impossible for a man to hate himself, properly speaking.
But accidentally it happens that a man hates himself: and this in two ways. First, on the part of the good which a man wills to himself. For it happens sometimes that what is desired as good in some particular respect, is simply evil; and in this way, a man accidentally wills evil to himself; and thus hates himself. Secondly, in regard to himself, to whom he wills good. For each thing is that which is predominant in it; wherefore the state is said to do what the king does, as if the king were the whole state. Now it is clear that man is principally the mind of man. And it happens that some men account themselves as being principally that which they are in their material and sensitive nature. Wherefore they love themselves according to what they take themselves to be, while they hate that which they really are, by desiring what is contrary to reason. And in both these ways, "he that loveth iniquity hateth" not only "his own soul," but also himself.
Wherefore the reply to the First Objection is evident.
Reply to Objection 2. No man wills and works evil to himself, except he apprehend it under the aspect of good. For even they who kill themselves, apprehend death itself as a good, considered as putting an end to some unhappiness or pain.
Reply to Objection 3. The miser hates something accidental to himself, but not for that reason does he hate himself: thus a sick man hates his sickness for the very reason that he loves himself. Or we may say that avarice makes man hateful to others, but not to himself. In fact, it is caused by inordinate self-love, in respect of which, man desires temporal goods for himself more than he should.
Source: Summa Theologica (New Advent)