Albert Barnes on Mark 5:15

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.

(a) "devil, and had" Isa 49:25, Col 1:13 (b) "they were afraid" Job 13:11, Psa 14:5, 2Ti 1:7