This is an interesting topic for the list! My understanding from the sermons and Bible studies on Mark 5:1-20 has been that the demons asked to be sent into the pigs because pigs were not (and are not now) butchered and eaten by Jews; it is against their faith to eat pork. The demons thus felt slyly "safe" in suggesting habitation in the swine, since these creatures would not be killed by any Jew of the Gerasene region. That is perhaps another reason that they were asking not to be sent away from the area (verse 10), to where pigs lives' might not be so sacred. However, the joke is on the demons. They were in a new "host", with different reactions and thought patterns than the tormented man. They could not "control" the actions of the pigs with the same finesse and experience as they had done with the man. Instead of breaking chains and handcuffs, screaming and gashing himself with stones (to release the demons?), the effect on the pigs was to run (in fear, in pain?), and unfortunately, the impulse to run into the water to escape the torments of the demons overcame their natural aversion to water.
I don't know, but I don't believe that pigs have the same thought processes about moral or ethical choices as we do, otherwise we would not have been granted stewardship in our relationship to them and God's creation in Genesis. I don't believe the action of the pigs was anything other than the compelling impulse response to a strong stimulus, as if they were stung by a cloud of bees, or were threatened by a feeling of fire.
What is more interesting to me on this topic is that this account also appears in Matthew 9:28-34, and Luke 8:26-39. A footnote in my Bible under the Matthew 9:29 text notes that the demons cried: "Why meddle with us, Son of God? Have you come to torture us *before the appointed time*?" (The emphasis is mine.) The footnote continues to say that the demons already recognize and feel the threat of Jesus even before the destruction of Satan's power at the end of the world, citing 1 Corinthians 15, 24f. In the Luke version, the demons plead to not be sent back to the abyss (Luke 8:31). What finally hit me is that in ALL these passages, the demons call Jesus the Son of God, before anyone else recognized him as such! With virtually all of Christ's miracles before and after this episode, he warns those who were healed or made clean not to speak of it (eg. See Mark 5:43). When people saw the awesome power of Christ, as in the removal of the demons, they were fearful and asked him to leave (Mark 5:17, Matthew 9:34, Luke 8:37). It was counterproductive for His ministry for people to brag about his powers, as this just scared those who heard of it, and threatened the leaders of the church.
My pastor always reminds us in this passage of the audience who saw what Jesus had done. Early Jews would have been very satisfied with the expulsion of the demons to something that in their belief was worthless; it would have been a good punch line to the joke on the demons. With their request to go to the pigs, they intended to get safety, what they got was their own destruction. When their host was destroyed, so were they.
Anyway, it's another theory.
Your sister in Christ,