After Jesus rebukes the storm and calms its violence, he sets foot in the country of the Gadarenes where rebukes two demon-possessed men and calms their violence. Now you might be wondering, why are there two men? Don't I recall hearing this story before, except it involved only one man? Yes, Mark tells that story in 5:1-20, where he also goes into greater depth about Jesus' interaction with the demon-possessed man than Matthew
Here's where a lot of people start freaking out and accusing the Bible of "contradictions." But you don't have to go there, if you just understand that the Gospel writers are not making the same claim as CNN news reporters. They are not claiming to report "just the facts" and let the audience decide what's important. Each Gospel writer either includes or omits material depending on the point he is trying to make (and I would argue that CNN news reporters do the same thing, they just don't like to admit it). So Matthew tells a story of two demon-possessed men with considerably less detail about Jesus' personal interaction with the demons and more emphasis on the miracle itself. The account is fairly brief. Mark, on the other hand, focuses on Jesus' relationship with one of the demoniacs as if he were the only person in view. And so even though there may have been two men present, Mark's interest is in the journey of just this one man in particular: his personal transformation, his desire to follow Jesus, his commission from the Lord to testify of the mercy he received. It's obvious the story is told in great detail so that we could feel a sympathy and connection with this healed man.
So anyhow, that's my theory about the "contradiction" between Matthew and Mark, for what it's worth. I'm sure there are other plausible theories out there you might also want to consider.
Getting back to our passage, Jesus encounters two demoniacs on the road who immediately recognize him and start wigging out. Jesus never says a word about hell or everlasting torment; they bring it up themselves. "What do we have to do with you, Son of God? Have you come to torment us before the time?" Demons are a doomed race and they know it. Even while they are doing their mischief they understand that their days are numbered, and so when Jesus shows up earlier than their eschatological timetable predicted, they're like, "What? So soon? We thought we had more time! Give us more time!" Their cowering fear and subservient posture before the Lord tell us that we who have a personal relationship with Jesus have nothing to fear from demons. Before Jesus they are as overmatched as a colony of ants before a can of Raid. I'm not saying demons aren't to be respected. As creatures they are far superior to human beings in power, intelligence, mobility, longevity and perception. But Jesus is Lord over both humans and demons, and if you hide yourself in him you've got nothing to worry about.
These demons are terrified that Jesus is going to cast them out of the men and into the depths of hell, so they're begging him, "Send us into the swine! Send us into the swine!" and Jesus says, "Okay, fine." Apparently the swine could handle them no better than the men, for they immediately rush down the bank and drown themselves. I can't imagine that the demons are destroyed as a result; they probably just escape off somewhere, looking for someone else to possess. So what is going on here? When Jesus casts the demons into the swine I don't think he is trying to destroy them, but rather is acknowledging that the time to send them into the eternal fires of hell has not yet come. The demons, by asking to be sent into the pigs, are seeking an escape pod out of the presence of Jesus and back into the world. He willingly grants them that escape. It's kind of like those action hero movies where the hero shows his complete dominance over his opponent not by destroying him but having mercy on him for a time. Jesus has that kind of dominance over the demonic world.
The herdsmen witness the whole thing, they rush into the city to report everything that happened, and the whole city comes out to meet Jesus to thank him, praise him and fall down at his feet. Right? Wrong. "The whole city came out to meet Jesus, and when they saw him they entreated him to depart from their region." Apparently these folks subscribe to the "better the devil you know than the devil you don't" philosophy. They'd rather have a bunch of demoniacs running around than a stranger more powerful than demons on the loose. Like the demons, these townspeople feel overmatched by Jesus; and like the demons, they also plead to be released from his presence. Sadly, both the demon and human inhabitants of the Gadarenes have a great deal in common.