John Chrysostom

If the devil is a murderer, it is clear that the demons who serve him are murderers, too. What Christ did has taught you this lesson. At any rate, he gave the demons leave to enter into the herd of swine and the demons drove the whole herd down the cliff and drowned them. He did this so that you might know that the demons would have done the same thing to human beings and would have drowned them if God had allowed them to do so. But he restrained the demons, stopped them, and permitted them to do no such thing. …

Against the Jews, Homily 8 ↗

Theophylact of Ohrid

He agrees to this. Since our life is a warfare, the Lord did not wish to remove the demons from it, so that through their struggle against us they might make us more skilled. He permits them to enter the swine so that we may know that just as they did not spare the swine, so they would not have spared that man either, had the power of God not preserved him. For the demons, being hostile to us, would immediately destroy us if God did not protect us. …

Commentary on Mark ↗

Jerome

But the old man declared to them how the devil, for men's sake, seizes even beasts of burden; that he is inflamed by such intense hatred for men that he desires to destroy not only them but what belongs to them. As an illustration of this he added the fact that before he was permitted to try the saintly Job, he made an end of all his substance. …

The Life Of S. Hilarion 23 ↗

Bede

And the unclean spirits, coming out, entered into the pigs, and with great force, the herd rushed into the sea, about two thousand of them, and were drowned in the sea. This signifies that once the Church is glorified and the people of the Gentiles are freed from the domination of demons, those who refused to believe in Christ engage in their sacrilegious rites in secluded places, submerged in blind and profound curiosity. …

On the Gospel of Mark ↗