Whereas God’s presence is there, it is qualitatively different than it is elsewhere in creation, and due to that difference, it is more merciful to those who chose Hell than it would be for them to exist for an eternity in the full presence of the Glory of the God-head in Heaven. There are many passages in scripture that discuss the nature of the Glory of God and that support this notion. There is also some support for the notion that it is merciful to the believer for the unregenerate to go to Hell rather than everyone winding up in heaven (the Universalist heresy).
Hell exists. End of Story. Some of the few passages that discuss Hell are in terms of Gehenna (the garbage heap that was perpetually burning outside of Jerusalem, also seen as the Lake of Fire that was made in reservation for the Devil and his angels. If scripture is to be believed, those who are currently in torment in Sheol will join them after the white throne judgment). Others are just about the Grave in terms of either Hades or Sheol. Hades is the Greek term that roughly equates to Sheol. Sheol is the term for “The Pit,” and is frequently referenced in the Psalms.
In its essence, Hell is torment, as seen in Luke 16 in the story of the Rich Man and Lazarus, whether a literal flame or a sense of loneliness or an eternal loop of one’s favorite sin here on earth but without any satisfaction at the conclusion of it. The issue is that the latter two visions do not appear in scripture, other than in a few places where it refers to Hell as a place of darkness, which could potentially support the idea of loneliness. Despite the absence of such conjecture in scripture, it is possible to infer these conclusions based on the information scripture does provide.
How can there be mercy in this? God’s Glory. The glory of God is something that cannot be reduced to adequate terms in any human language, but there are passages that discuss it throughout scripture, and typically with imagery of burning fire, unapproachable light, and sometimes as a perfume or something radiantly beautiful.
God did not show himself fully to Moses on Sinai because he knew that his glory would kill Moses due to the nature of Sin, so God hid him in the cleft of the rock and covered his face with his hand that Moses would not see his face, but only his back as he passed. That lesser piece of God’s glory caused Moses’ face to glow for over a week; Moses had to veil his face because the Israelites could not suffer to see his face because it was shining so brightly. The burning bush is another picture of God’s glory, and that is the way that a fire could not consume a living bush, and indeed leave it green. Think of the pillars of cloud and fire during the Exodus, those were each manifestations of his Glory above the tabernacle. Indeed, his glory was why the Holy of Holies was inaccessible except for to the High Priest on the Day of Atonement; if any others dared to enter, they would die.
The same can be said for the Ark of the Covenant itself: As it was recounted in the time of the Judges, Saul and David understood that the ark could not be touched without death resulting. The Glory of God is also why when the ark was captured, the gods of the Philistines fell prostrate before it when placed in their temples. The glory of God will not suffer sin to be near it, because it is his perfection, his holiness and ultimate Love encapsulated (for lack of a better way to put it).
In 2nd Thessalonians 1:7, Jesus is shown as a blazing fire on Judgment Day. Indeed, even in Saul’s conversion experience on the Damascus Road, Jesus appeared as in a blinding light. This is a manifestation of God’s glory, as was the transfiguration in the Gospels of Matthew, Mark, and Luke. Those who do not ascribe God the glory due him because of who he is arguably choose to go to Hell, and finally to the Lake of Fire. That choice is merciful in and of itself, because it gives us the ability to accept grace and be prepared for heaven, or to reject it completely and to not be able to bear the weight and heat of His unfathomable beauty, and to go to the place where it is not the same as the rest of creation, and nowhere near the intensity that exists in Heaven.
Hell is that place, not that His glory is not present there at all, but indeed it is what manifests as the torment to those who are there for eternity. It might be said that Hell is not as bad for those who are there as Heaven would be for that same demographic. The intensity and quality of God’s glory in heaven is too great for one who is unprepared through a life of accepted tenderness here on earth to bear. It is too bright, much like the illustration of the Omnibus that C.S. Lewis uses in The Great Divorce, for those denizens of Hell to bear.
Mercy In Hell? Yes. By allowing man to choose not to be with him, God grants mercy. The Father’s heart is that all men should be saved. However, there are those who, according to Paul, were created for destruction. How can this be? We may never fully understand. Hell is merciful in the sense that His glory is of a different intensity than in Heaven, and the suffering of the damned is somehow different if not lesser than if they had their free will revoked when they died and were forced to worship God for eternity in the seat of his Glory in heaven. Hell is a form of mercy.