First prove that Demon's actually exist, then we can move to the question you ask.
Because I believe that the Scriptures is Truth and that God is the Author of the Bible, I believe His Word.
Ephesians 6:12
“For we wrestle not against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this world, against spiritual wickedness in high
places.”
Jesus also cast out demons from different people.
Luke 9:42
“And as he was yet a coming, the demon threw him down, and tear
him. And Jesus rebuked the unclean spirit, and healed the child, and delivered him again to his father.”
Mark 5:15 Context
12And all the demons besought him, saying, Send us into the swine, that we may enter into them.
13And forthwith Jesus gave them leave. And the unclean spirits went out, and entered into the swine: and the herd ran violently down a steep place into the sea, (they were about two thousand;) and were choked in the sea.
14And they that fed the swine fled, and told
it in the city, and in the country. And they went out to see what it was that was done.
15And they come to Jesus, and see him that was possessed with the demon, and had the legion, sitting, and clothed, and in his right mind: and they were afraid. 16And they that saw
it told them how it befell to him that was possessed with the demon, and
also concerning the swine.
17And they began to pray him to depart out of their coasts.
18And when he was come into the ship, he that had been possessed with the demon prayed him that he might be with him.
and more.....