It strikes me as strange in a way to awaken someone to put them to death the second time. They were already dead, if the aim was for them to die and cease to exist why not leave them in the dead (and presumably nonexistent) state they are already in?
Not sure how much of this thread you've looked through, but this was actually already addressed by me in post #38 and by James' in post #50.
There is a judgment day which lasts through the thousand year reign of Jesus Christ when people will be judged for the deeds they do during that reign, this happens after the resurrection of the righteous and the unrighteous. People will not be judged for the sins they committed before they died because their death paid for those sins, but they will be judged for what they do during the thousand years Jesus rules over the earth. For Christians, the death of Christ paid for all sins (past, present and future) and they are saved from the second death and part of the first Resurrection.
Shortly after the millennium (Rev 20:5), every man, woman, and child who did not receive an unencumbered opportunity to know Christ, in this age, will be resurrected back to physical life and be given a period of time (Isa 65:20), with the veil of deception removed (Isa 25:7) to accept or reject Him. Those who reject Him will be cast into the lake of fire and die the second death and simply cease to exist. If there is a second death, logic would dictate there had to be a first, right?
Also you make the point about why would a merciful Lord put people in fire for eternity, I think in a way the same question could be asked, why would a merciful Lord execute people by burning? Surely there are more kind methods of execution than fire?
What type of "execution" would you consider more appropriate for the wicked? Electric chair? Gas chamber? Firing squad? Oh, wait....
Seriously though, we don't know exactly what the wicked will experience at the moment they are condemned. All we know is that the wicked are "disposed of" in the Lake of fire. We don't even really know that the Lake of Fire is "literal" lake of burning sulfur. It does come from the book of Revelation after all. The whole thing could very well just be a symbolic metaphor for "the second death". Perhaps God WILL be merciful on them (even if they don't deserve it). Regardless, it would be very unmerciful for the righteous to allow the unrighteous to corrupt them in God's kingdom. A kingdom that is supposed to be free of death, pain, suffering, and sin. These things aren't supposed to exist in the new creation, therefore the wicked cannot exist in the new creation. The wicked must die in order for there to be peace in God's kingdom. I think a lot of people would argue that they don't deserve God's mercy at that point, although I sense that they will get it anyway.
Consider this in light of for example; a Hindu who made great contributions to medical science that saved many lives, but didn't believe in Jesus so this Hindu in addition to whatever suffering is involved in his first death, is then raised up to be burned to death. Can you see how the same question you put to others regarding the actions of a merciful Lord still pertains to your interpretation of hell to a certain extent?
Again, see posts 38 and 50.
Remember, we are not all judged by the same standard. Christians are judged to a higher standard than someone who has never known Christ. People are ultimately held accountable to what they know, not what they don't know.
Luke 12:47-48 (Parable of the master and faithful servant)
47 “And a servant who knows what the master wants, but isn’t prepared and doesn’t carry out those instructions,
will be severely punished. 48 But someone who does not know, and then does something wrong,
will be punished only lightly. When someone has been given much,
much will be required in return; and when someone has been entrusted with much,
even more will be required.
Romans 2:14-16
14 Even Gentiles,
who do not have God’s written law, show that they know his law when they instinctively obey it, even without having heard it. 15 They demonstrate that God’s law is written in their hearts,
for their own conscience and thoughts either accuse them or tell them they are doing right. 16 And this is the message I proclaim—that the day is coming when God, through Christ Jesus,
will judge everyone’s secret life.
Revelation 20:12
12 I saw the dead, both great and small, standing before God’s throne. And the books were opened, including the Book of Life.
And the dead were judged according to what they had done, as recorded in the books.