Objection 1. It would seem that no other special sins, besides pride and avarice, should be called capital. Because "the head seems to be to an animal, what the root is to a plant," as stated in De Anima ii, text. 38: for the roots are like a mouth. If therefore covetousness is called the "root of all evils," it seems that it alone, and no other sin, should be called a capital vice.
Objection 2. Further, the head bears a certain relation of order to the other members, in so far as sensation and movement follow from the head. But sin implies privation of order. Therefore sin has not the character of head: so that no sins should be called capital.
Objection 3. Further, capital crimes are those which receive capital punishment. But every kind of sin comprises some that are punished thus. Therefore the capital sins are not certain specific sins.
On the contrary, Gregory (Moral. xxxi, 17) enumerates certain special vices under the name of capital.
I answer that, The word capital is derived from "caput" [a head]. Now the head, properly speaking, is that part of an animal's body, which is the principle and director of the whole animal. Hence, metaphorically speaking, every principle is called a head, and even men who direct and govern others are called heads. Accordingly a capital vice is so called, in the first place, from "head" taken in the proper sense, and thus the name "capital" is given to a sin for which capital punishment is inflicted. It is not in this sense that we are now speaking of capital sins, but in another sense, in which the term "capital" is derived from head, taken metaphorically for a principle or director of others. In this way a capital vice is one from which other vices arise, chiefly by being their final cause, which origin is formal, as stated above (Question 72, Article 6). Wherefore a capital vice is not only the principle of others, but is also their director and, in a way, their leader: because the art or habit, to which the end belongs, is always the principle and the commander in matters concerning the means. Hence Gregory (Moral. xxxi, 17) compares these capital vices to the "leaders of an army."
Reply to Objection 1. The term "capital" is taken from "caput" and applied to something connected with, or partaking of the head, as having some property thereof, but not as being the head taken literally. And therefore the capital vices are not only those which have the character of primary origin, as covetousness which is called the "root," and pride which is called the beginning, but also those which have the character of proximate origin in respect of several sins.
Reply to Objection 2. Sin lacks order in so far as it turns away from God, for in this respect it is an evil, and evil, according to Augustine (De Natura Boni iv), is "the privation of mode, species and order." But in so far as sin implies a turning to something, it regards some good: wherefore, in this respect, there can be order in sin.
Reply to Objection 3. This objection considers capital sin as so called from the punishment it deserves, in which sense we are not taking it here.
Source: Summa Theologica (New Advent)