The reasons for Christ's second coming are to judge those who know not God, to judge those who obey not the gospel (2 Thess 1:8) and to judge those obey the gospel (Matt 25:31-46). In Revelation 20:11-15, the sea gave up the dead which were in it. The sea refers to the wicked people of the world who committed all kinds of wickedness and escaped justice. These are the people who enthroned Hitler and supported his murdering 6,000 jews. Also, these are the people who supported the water beast which is liken to lion, leopard and bear (Rev 13:1-10). These people committed secret murders, secret envies, secret immoralities and secret hates. Christ's second coming is their payday.
In Revelation 20:13, death gave up the dead which were in it. At Christ's second coming death will be judged for its cruelty, its bondage and its dominion over mankind. Thus, death shall be no more, and man shall become incorruptible, immortal. Finally, Hades gave up the dead which were in it (Rev 20:13). At Christ's second coming he judges the Hadean world where all the dead from the earth go. Those who are in Abraham's bosom in Hades will return to heaven with the Lord; those in the place of punishment will be assigned to the eternal furnace of fire.