Does Allah also destroy humans completely as if some of them had never existed (just vaporized)?
If so, does this mean that a human being is better off being completely destroyed than being tormented in hell for his/her sins?
The answer is a big NO...
Humans will never be destroyed(after ressurection). They will taste death once and be resurrected after which they will either go to heaven or hell. Obviously, being destroyed completely is better than an eternal torment.
Verily, We have warned you of a near torment, the Day when man will see that (the deeds) which his hands have sent forth, and the disbeliever will say: "Woe to me! Would that I were dust!
The above verse shows that every disbeliever would wish for complete destruction , but nowhere in the Qur'an does Allah mention of complete destruction as death will be forbidden for humans and jinns.
Though Allah does mention about some people who neither go to hell or heaven referred to as the Companions of The Elevations (al-aaraf):
And between them will be a barrier screen and on Al-A'raf (a wall with elevated places) will be men (whose good and evil deeds would be equal in scale), who would recognise all (of the Paradise and Hell people), by their marks (the dwellers of Paradise by their white faces and the dwellers of Hell by their black faces), they will call out to the dwellers of Paradise, "Salamun 'Alaikum" (peace be on you), and at that time they (men on Al-A'raf) will not yet have entered it (Paradise), but they will hope to enter (it) with certainty.
And when their eyes will be turned towards the dwellers of the Fire, they will say: "Our Lord! Place us not with the people who are Zalimun (polytheists and wrong-doers)."
And the men on Al-A'raf (the wall) will call unto the men whom they would recognise by their marks, saying: "Of what benefit to you were your great numbers (and hoards of wealth), and your arrogance against Faith?"
Are they those, of whom you swore that Allah would never show them mercy. (Behold! It has been said to them): "Enter Paradise, no fear shall be on you, nor shall you grieve."
You can see that they also will enter heaven so in the end there's no intermediate destination..
Will disbelievers go to heaven??
Verily, those who belie Our Ayat (proofs, evidences, verses, lessons, signs, revelations, etc.) and treat them with arrogance, for them the gates of heaven will not be opened, and they will not enter Paradise until the camel goes through the eye of the needle (which is impossible). Thus do We recompense the Mujrimun (criminals, polytheists, sinners, etc.).
And Allah knows best.. Baarakallahu feekum..
Allah can do this but does not, the purpose of this life is a test, the purpose of the human being is to worship, in the end each one of us would be judged according to their actions, those that succeeded and followed the guidance and believed would go to Jannah, as for those who disbelieved and fought against the guidance, why would Allah provide a easy way out?
We could argue that yes this would be better, but the fact is no where in the Quran or Sunnah does it say that Allah would destroy a persons total existence, and in fact it is the otherway around that each person will have the results of what they did in this world and no one can escape those results, and the disbelievers would be trying to find easy way out, whether it is wishing to be dust (As was mentioned in the other answer) or asking Allah to return them to this world so they can do good, which they would not.
In simple terms, there is no escape from the day of Judgement, Allah will judge accordingly those that deserve heaven go there, those that deserve hell go there.
Note: We humans are not part of Allah, nor is Allah a part of us, The Creator and the creation are two separate things, The Creator is everlasting and does what he wills, the creation is not everlasting and is bound by the law Allah set for it.