Death is not the same as being annihilated.
There are two ways of defining dead.
- the lack of physical life - not breathing, no consciousness, no pulse and eventually, no body. Just 'dust' remains and maybe some teeth! From this death, life is again possible by a resurrection at the time of God's choosing.
- Spiritually dead is the state of being dead to sin, even if you are still breathing. Like the term, 'dead man walking' - referring to a prisoner who is sentenced to death, and is on his way to dying - he is essentially dead already! The spiritually dead includes all men who ever lived, except Jesus. All are essentially waiting for the execution of the penalty handed to Adam - "the penalty of sin is death". So even while they live, they are dead. These too can be raised to new life - physical or spiritual, by God.
Annihilation is the removal (or expiry) of the resurrection option. They simply no longer exist in any form. This can only happen after the second death mentioned in Rev 20. This is not 'dead' in either of the previously noted states - it is irreversible.
The biblical basis for this understanding is not based on what scripture explicitly says this, but what it does not say.
The dead will not live, the departed spirits will not rise; Therefore You have punished and destroyed them, And You have eliminated all remembrance of them. Is 26:14
The context of this passage is not end-time, post second-death; it is a song about life and troubles and the God who provides.
What we do have is no mention of any evil person being given eternal life. None! How can the wicked be tormented forever in some kind of demented, spiteful God scenario who takes delight in giving life to all and letting the wicked suffer for eternity!
As explained here, eternity does not have mean 'forever' and punishment does not mean torture or even death.
In the absence of scripture and the biased 'hellfire' interpretations like Matt 25:46, it seems many would rather make up what they want and pursue a theology not of the bible, but of men, desperate to justify their jobs, influence and income.
Certainly the theology of an eternal hellfire for the wicked is intended to extinguish the true revelation of the Judgement and the true God who is Saviour first and Destroyer second. Also addressed in the link provided.
After the 'second death' Rev 20:14, the next revelation in Rev 21 is,
Then I saw a new heaven and a new earth; for the first heaven and the first earth passed away, and there is no longer any sea...
This is a time when all physical life is gone. All remaining have spirit lives and life eternal.
God Himself will be among them, 4 and He will wipe away every tear from their eyes; and there will no longer be any death; there will no longer be any mourning, or crying, or pain; the first things have passed away.” v3
At this point, God's plan of salvation has been completed - all created humanity that has accepted Christ as their Saviour has been changed to spirit life.
What we are also told is that 'God is love' - torturing a broken and deceived humanity is not part of His plan. He will however hold to account the perpetrator of evil in ways that we are not privy to.