Charles Ellicott Commentary Matthew 8:31

"And the demons besought him, saying, If thou cast us out, send us away into the herd of swine." — Matthew 8:31 (ASV)

So the demons begged him. Mark records their words as a plea that He should not send them out of the country, or district, where they were. Similarly, Luke reports that they begged Him not to command them to go out into the deep—that is, the abyss, the bottomless pit mentioned in Revelation 9:1–2 and 9:11.

The man's words are the same as those of the demons with whom he identifies himself. He shrinks from the thought of wandering in dry places, seeking rest, and finding none (Matthew 12:43), or being compelled to flee, like Asmodeus, into the farthest parts of Egypt . Worst of all, he fears being sent into the “abyss,” which was the ultimate doom of evil.

And so, speaking as one with them, he suggests another alternative: If you cast us out, send us into the herd of swine. The implication is clear: If the power to terrify and disturb humans is taken from us, let us, at least, retain the power to destroy animals.