Objection 1. It would seem that the devil is not the head of the wicked. For it belongs to the head to diffuse sense and movement into the members, as a gloss says, on Ephesians 1:22, "And made Him head," etc. But the devil has no power of spreading the evil of sin, which proceeds from the will of the sinner. Therefore the devil cannot be called the head of the wicked.
Objection 2. Further, by every sin a man is made evil. But not every sin is from the devil; and this is plain as regards the demons, who did not sin through the persuasion of another; so likewise not every sin of man proceeds from the devil, for it is said (De Eccles. Dogm. lxxxii): "Not all our wicked thoughts are always raised up by the suggestion of the devil; but sometimes they spring from the movement of our will." Therefore the devil is not the head of all the wicked.
Objection 3. Further, one head is placed on one body. But the whole multitude of the wicked do not seem to have anything in which they are united, for evil is contrary to evil and springs from divers defects, as Dionysius says (Div. Nom. iv). Therefore the devil cannot be called the head of all the wicked.
On the contrary, A gloss [St. Gregory, Moral. xiv] on Job 18:17, "Let the memory of him perish from the earth," says: "This is said of every evil one, yet so as to be referred to the head," i.e. the devil.
I answer that, As was said above (Article 6), the head not only influences the members interiorly, but also governs them exteriorly, directing their actions to an end. Hence it may be said that anyone is the head of a multitude, either as regards both, i.e. by interior influence and exterior governance, and thus Christ is the Head of the Church, as was stated (6); or as regards exterior governance, and thus every prince or prelate is head of the multitude subject to him. And in this way the devil is head of all the wicked. For, as is written (Job 41:25): "He is king over all the children of pride." Now it belongs to a governor to lead those whom he governs to their end. But the end of the devil is the aversion of the rational creature from God; hence from the beginning he has endeavored to lead man from obeying the Divine precept. But aversion from God has the nature of an end, inasmuch as it is sought for under the appearance of liberty, according to Jeremiah 2:20: "Of old time thou hast broken my yoke, thou hast burst my bands, and thou saidst, 'I will not serve.'" Hence, inasmuch as some are brought to this end by sinning, they fall under the rule and government of the devil, and therefore he is called their head.
Reply to Objection 1. Although the devil does not influence the rational mind interiorly, yet he beguiles it to evil by persuasion.
Reply to Objection 2. A governor does not always suggest to his subjects to obey his will; but proposes to all the sign of his will, in consequence of which some are incited by inducement, and some of their own free-will, as is plain in the leader of an army, whose standard all the soldiers follow, though no one persuades them. Therefore in the same way, the first sin of the devil, who "sinneth from the beginning" (1 John 3:8), is held out to all to be followed, and some imitate at his suggestion, and some of their own will without any suggestion. And hence the devil is the head of all the wicked, inasmuch as they imitate Him, according to Wisdom 2:24-25: "By the envy of the devil, death came into the world. And they follow him that are of his side."
Reply to Objection 3. All sins agree in aversion from God, although they differ by conversion to different changeable goods.
Source: Summa Theologica (New Advent)