Jesus is showing compassion to the man
The relationship between Jesus and demons is complicated as presented in Mark. On the one hand, Jesus has authority over them, and on the other, they still cause problems for him. Take his first encounter with a demon recorded in Mark 1:23-26 (ESV):
And immediately there was in their synagogue a man with an unclean spirit. And he cried out, “What have you to do with us, Jesus of Nazareth? Have you come to destroy us? I know who you are—the Holy One of God.” But Jesus rebuked him, saying, “Be silent, and come out of him!” And the unclean spirit, convulsing him and crying out with a loud voice, came out of him.
Jesus answers the charge that his powers are from Beelzebul in Mark 3:23-27 (ESV):
And he called them to him and said to them in parables, “How can Satan cast out Satan? If a kingdom is divided against itself, that kingdom cannot stand. And if a house is divided against itself, that house will not be able to stand. And if Satan has risen up against himself and is divided, he cannot stand, but is coming to an end. But no one can enter a strong man's house and plunder his goods, unless he first binds the strong man. Then indeed he may plunder his house.”
I won't give a complete survey of demons and unclean spirits in the Gospels, but if you do the exercise, you'll see that:
- Demons are a problem in the northern region from the Sea of Galilee to Tyre and Sidon. They don't show up in and around Jerusalem.
- Jesus always casts out demons.
- Jesus often interacts directly with demons before dispatching them.
- When Jesus casts out a demon, there usually is no mention of what happens to the demon. Jesus seems to be able to completely eliminate them.
Since there were herds of pigs, the region mentioned in Mark 5 ("the country of the Gerasenes") must have been Gentile. Demons operated best in Gentile regions (extrapolated from point 1. above), so what the demons asked was to not be cast out of the happy-hunting ground of that country. They would have preferred to stay in the man, but they knew he would cast them out (from 2).
Mark emphasizes that the man came to Jesus and not the other way around ("when Jesus had stepped out of the boat, immediately there met him out of the tombs a man with an unclean spirit" and "And when he saw Jesus from afar, he ran and fell down before him."). Once there, however, the demons seem to take control of the interaction. They know what is about to happen to them (point 4) and take the opportunity (point 3) to attempt a negotiation. But the negotiation is a bit like the negotiation between the police and a man being evicted—he might ask to be able to walk out of the house rather than being dragged out. No matter what, the demons are not getting what they would really like to have: free reign over the man.
But what about the effect on the man? It seems there were two options for how the exorcism could go down:
- Legion disappears without a trace.
- Legion is allowed to leave in spectacular fashion.
Remember that the man was insanely violent and scary (Mark 5:3-4 ESV):
He lived among the tombs. And no one could bind him anymore, not even with a chain, for he had often been bound with shackles and chains, but he wrenched the chains apart, and he broke the shackles in pieces. No one had the strength to subdue him.
Afterward, the people didn't know what to make of him (Mark 5:15 ESV):
And they came to Jesus and saw the demon-possessed man, the one who had had the legion, sitting there, clothed and in his right mind, and they were afraid.
Jesus hadn't been operating in that country and it doesn't seem like the people knew who he was or what he was capable of. So I read their fear as arising from not knowing what happened to the powerful demons. Like going into a haunted house and everything suddenly gets quiet, the fear actually increases in that situation. But the people soon learned what had happened to the demons (Mark 5:16-17 ESV):
And those who had seen it described to them what had happened to the demon-possessed man and to the pigs. And they began to beg Jesus to depart from their region.
Now it seems that the man was able to live a more normal life after that. At least part of the reason might be that everyone knew his demons were destroyed. There was a considerable economic cost, but the blame for that seems to have rested solely on Jesus. When Jesus summarized the incident, he emphasized the Lord's mercy toward the man (Mark 5:19 ESV):
“Go home to your friends and tell them how much the Lord has done for you, and how he has had mercy on you.”
A passage in Luke 11:24-26 (ESV) seems to be related to this incident:
“When the unclean spirit has gone out of a person, it passes through waterless places seeking rest, and finding none it says, ‘I will return to my house from which I came.’ And when it comes, it finds the house swept and put in order. Then it goes and brings seven other spirits more evil than itself, and they enter and dwell there. And the last state of that person is worse than the first.”
Perhaps the demons were trying to avoid passing through "waterless places".