What does the account of pigs and demons mean? Luke 8
  
  31 And they begged Jesus repeatedly not to order them to go into the
  Abyss.
  
  32 A large herd of pigs was feeding there on the hillside. The demons
  begged Jesus to let them go into the pigs, and he gave them
  permission. 33 When the demons came out of the man, they went into the
  pigs, and the herd rushed down the steep bank into the lake and was
  drowned.



Why did the demons need to go inside the pigs? Would that be the only way to avoid abyss? Why not request to roam around in a desert or sit on a tree?
Why then did they kill the pigs? I thought they wanted to live in a body.
Why did Jesus send them into pigs instead of sending to abyss where they belonged?
Am I to make some spiritual/moral sense of this story or is it just a historical record?
In the case of the devils wanting to have permission to enter the pigs rather than be cast into no particular place it seems to be under a simple desire of destruction. The demons are enraged to destroy and in this story we clearly see the bounds of the destruction they seek are under the power of God. They are tied to a divine leash, not being permitted to destroy without boundaries.

The reason why Jesus actually permits them might be as an opportune time to prove to those watching that the demons were not imaginary but as they left the man who then had peace of mind, external evidence showed that they were real by immediately destroying the pigs.

Furthermore we see the curse for sin brought on man includes a curse upon nature and even animals. Jesus did not make his primary mission to protect the rights of animals or upholding good environmental practices but to put an end to the power of the Devil by his death:


  Since the children have flesh and blood, he too shared in their humanity so that by his death he might break the power of him who holds the power of death—that is, the devil —(NIV Hebrews 2:14)