Matthew 8:29 "
It is helpful to understand what the demons meant by "did you come here to torment us before the appointed time?" As you probably remember, Revelation 20:1-3 says of Jesus: “I saw an angel coming down out of heaven with the key of the abyss and a great chain in his hand. He seized the dragon, the original serpent, who is the Devil and Satan, and bound him for 1,000 years. And he hurled him into the abyss and shut it and sealed it over him, so that he would not mislead the nations anymore until the 1,000 years were ended. After this he must be released for a little while."
So the demons were referring to the future time that they know and recognized as their imprisonment in the "abyss." Granted, these are spirit creatures we are talking about, so no physical holding tank could restrain them. But God knew we as physical beings would not understand spiritual things unless they were put in a frame of reference we humans could relate to. So whatever this "abyss" is, it is some sort of restraint that prevents the demons from exerting their influence.
But how do the words “torture” and “torment” play into this? Regarding the original Greek word used here, consider this article: http://wol.jw.org/en/wol/d/r1/lp-e/1200004454
Here is a reasoning point that may help: Just as a holding cell, or even possibly solitary confinement might be a "torture" all of and in itself, so likewise restraining the demons inside the abyss is all by itself a torture. (Even a child being punished by being sent to his room or to sit in a corner knows how torturous being grounded can be.) Those who have been in prisons can relate to the mental (psychological and emotional) anguish just being in prison can have. Even if no actual physical torture is being applied, just the act of restraint, the inability to express free will, the inability to enjoy life can be torture.
So things we humans associate with physical torture would not affect a spirit creature. But more importantly is the principle at Jeremiah 19:5 and James 1:13. That is to say, God would never use this sort of torture on anyone.