Albert Barnes Commentary Mark 5:15

"And they come to Jesus, and behold him that was possessed with demons sitting, clothed and in his right mind, [even] him that had the legion: and they were afraid." — Mark 5:15 (ASV)

Sitting, and clothed, and in his right mind. There could be no doubt of the reality of this miracle. The man had been well known. He had long dwelt among the tombs, an object of terror and alarm. To see him all at once peaceful, and calm, and rational, was proof that it was the power of God only that had done it.

They were afraid. They were awed, as in the presence of God. The word does not mean here that they feared that any evil would happen to them, but that they were affected with awe; they felt that God was there; they were struck with astonishment at what Jesus had done.