Jesus did not spare the demons. Possessing the pigs was a humiliation for the demons. Then, the pigs immediately drowned themselves. The demons had to move on to their inevitable fate.
I think in one of the letters of St. Paul, He mentions something to the effect that even the angels didn’t condemn demons, “May God Rebuke you”.
An apologist here would know the chapter and verse.
It may have been an act of ‘mercy’ (so to speak).
While on the outside it looks like Jesus is “cooperating” with the demons, he is not. No plan of theirs stops the Gospel from spreading and Christ’s healing ministry to come to a stop. The action of the demons leads to the destruction of a herd of pigs and initial rejection of Jesus by the people of the area, but the situation turns around to Jesus’ advantage and that of the people in the end.
Jesus’ actions save the man, but the actions of the demons in trying to remain in the temporal world cause the pigs to rush over a cliff and kill themselves. This causes no little stir among the inhabitants that ask Jesus to leave (this may have been what the demons were trying to accomplish by being released into the pigs). But upon Jesus’ return to the area Christ’s miracle seems to have had the opposite affect as the people in the vicinity now welcome him with open arms.–Note Matthew 14:34-36 where the people now bring their sick to be healed by Jesus upon his return.
It should be noted that when one finds that the Bible’s details don’t match (compare this account in Mark with what one finds in in Matthew 8:28-34 and Luke 8:26-39), it appears that the writers are employing an ancient narrative device. In modern times we have different techniques we use when we want people to pay attention to facts over the details in a story, but in the past the writers often did this by making the setting of a story fanciful or ambiguous. The books of Judith and Jonah do this. The truth of the account is not in how many men Jesus healed or what the name of the place was in which these things happened (it changes in the manuscripts among which there is no agreement), but in what Jesus did. Already before his Passion the spirit world and the temporal world were under his control.
Most scholars agree that certain details about the situation are just not available (as the ambiguous narrative setting seems to suggest). What happened to the demons once the pigs died? They obviously lost their influence in the area as upon Jesus’ return the people are happy to see Christ. Did they eventually end up in the place they didn’t want to go? The account doesn’t say, and in face of the narrative device it appears that such answers are not part of God’s current revelation to humankind.
Again the story is that no demon can stop Jesus from bringing God’s Kingdom to the people. Even when people initially reject the Gospel message, given enough time things can change.