Interestingly, Jesus never rebukes demonised people because He had compassion on those whose lives were being blighted by Satan (Matthew 14:14).  On this occasion, Jesus spoke to the evil spirit and demanded its name.  His authoritative command forced evil to reveal its name and nature.  There was not one demon but many.  Jesus had intentionally selected a man who was terrorised by a legion of spirits - a seething pot of evil.  And Jesus had intentionally brought the disciples to see what Jesus would do and how He would do it.  As the future leaders of the church, their Master would demonstrate that He had power to heal the most wickedly corrupted person, and throw out Satan's forces.

Was it the man speaking, or the demons?  Jesus addressed the tormented person, who was robbed of his dignity and compelled into violence and self-mutilation (Mark 5:1-5).  By asking his name, Jesus was demanding that he revealed his character.  Answering, the man so identified himself with his controlling demons that he called himself Legion.  His security was so bound into their manipulating power that he dreaded being without them; perhaps they could be sent into the fields nearby but not too far away.   Although they tormented him, he thought he could not live without them.  Like many abused people, he believed that his identity was dependent on his tormentors, and did not want to let them go (Mark 1:23-26).

The next voice was demonic.  For demons to barter their fate with Jesus may seem rather odd.  But think of them as viruses: they can only live by inhabiting a host who is used as they propagate and is often destroyed in the process.  They would prefer the power-thrill of drowning pigs than being without something to terrorise.  Perhaps it was a measure of the number of demons exorcised by Jesus, that 2,000 pigs were so disturbed as to plunge into the lake.  But Jesus' permission enabled His trainee apostles to see how evil, evil is.  The sheer scale of this unprecedented mass destruction, and the evidence of the healed man (Mark 5:15), showed the extent of Satan's grip on this human life.  It also showed that Jesus had authority over the worst of evil, and complete power to release its prisoners (Isaiah 61:1).  

When Jesus speaks, He does so with powerful effect. We must be both encouraged and challenged.  Encouraged, that Jesus has far greater power than even the worst of evil; and challenged, that we are His ambassadors in an evil world to call those trapped by Satan to trust in the Lord Jesus Christ (Hebrews 2:14-15).  He has placed His Word into our hands, and His Spirit in our hearts, to be salt and light in a hostile world (Matthew 5:11-16).  This should spur us to trust Him in the apparently impossible situations of today and tomorrow, and to dare to apply His Word where evil is at its worst.