Would you like the evils of this world to be righted? More specifically would you like the evils and problems local to you sorted out? The chances are you answered yes; most people would; it seems logical. However the reality is that many people have an irrational desire to maintain the status quo even if the proposed change is clearly a good one. If you add to the equation that the proposed change is socially good, clearly morally right but potentially disadvantageous to the person being asked the question then the reality is that most of us would rather keep things biased in our favor.
In the case of the people of Decapolis we are told that they were afraid that the Lord had been able to heal the man with the devil. He had clearly been a famous part of local folk law anyone powerful enough to change that must himself be scary. I suspect the prospect of having to integrate this once scary character into their society was scary too. How many congregations would really, really want a reformed homicidal maniac in their midst?
I also wonder how many of them had little issues of their own? Perhaps not ones open enough to get you chained up in a graveyard but ones which a teacher of the power and might of the Lord might perceive and destroy. Fear that the life change that happened to the madman might happen to them could be real. However, my suspicion is that the real issue was the pigs.
Sometimes we gloss over numbers in scripture without really figuring upon what they mean. Two thousand swine is an incredibly large herd. It corresponds to over 400,000lbs of meat. This herd of swine must either have been the entire meat store for a very large area or it must have represented the primary income method for the village. And in one day the entire herd plunged off the edge of a cliff, in exchange for one person that none of them really knew. To put this into a modern perspective we would have to consider a small town built around a single major employer. Then that major employer closes the plant in that town in exchange for one criminally insane person being released from a local jail.
When we read this passage in Mark we typically condemn the people of Gadarenes very quickly. We believe we want a radical life changing relationship with the Lord. Typically however we place bounds upon what we want changed and how radical we want to be. We probably don't mind the cursing going, we can loose the beer and cigarettes, we fully understand the need for honesty and even, at least in principle, sexual purity. But do we really want a relationship that can loose us absolutely everything, for the benefit of someone else. Obviously we want souls saved, but how much are we willing to give up to see a soul saved? Typically we accept the Lord 'trimming the fat' but if the cost stretches to something important: we become afraid.
In conclusion: the people of Gadarenes were called upon to make a huge sacrifice for the benefit of one of their number. Although they saw and at least partially understood the power of God this challenge took them massively out of their comfort zone and they wanted God gone. The challenge to us as believers is this: How far out of our comfort zone are we prepared to let the Lord take us for the benefit of others?