Even though I believe this has been answered elsewhere, I'll go ahead and answer this based on the comments to the original question.
Before I do, I need to remind everyone of the narrow scope of this site: We are not focused on proving any particular thing to be true. We focus solely on what the teachings are. As such it would be against site guidelines to answer whether the eternal punishment in Hell is true, or if God was justified to create an eternal place of torment. We can only answer what is taught within Christianity.
I'm going to answer this from the standard protestant Apologetics answer, which shares common themes across denominational boundaries.
The question here, as focused by the comments is "can the belief in hell be justified against the U.N. convention of disallowing torture".
The obvious answer here is the one that God gave to Job when Job questioned him about his misery. "Who are we to question God?"
Job 38:4 (KJV)
Where were you when I laid the foundations of the earth? declare, if
you have understanding.
As explained by Matthew Henry's Concise Commentary
For the humbling of Job, God here shows him his ignorance, even
concerning the earth and the sea. As we cannot find fault with God's
work, so we need not fear concerning it. The works of his providence,
as well as the work of creation, never can be broken; and the work of
redemption is no less firm, of which Christ himself is both the
Foundation and the Corner-stone. The church stands as firm as the
In other words, God does not have to justify torture. (More on this later in the answer)
But that's not what you asked. What you specifically is how we humans can justify the belief in Hell when the U.N. has condemned it.
Again - short answer. We believe it because God said it's true. As pointed out by God in his discussion with Job (And by DJClayworth's comment), God is sovereign. God doesn't answer to us, we answer to Him.
We believe (at least most Christians do) that the Bible is God's inspired, inerrant Word.
Side note: Why we believe that would be the basis of a hundred other questions already answered on this site. And to make a distinction, that we accept this is not an implicit assumption, it is an established doctrine. Remember, we are not here to teach what is True, just what Christianity teaches, and it is undeniable that Christianity teaches that the Bible is God's word. Any attempt to steer this into a direction of whether it really is God's word is strictly not allowed in the site guidelines.
So that's it in a nutshell. We justify our belief because we trust God's word, and His word says it is so. We do not believe that God is accountable to the U.N. or any other organization/being, etc.
Addressing your qualifiers:
The satisfying answer should justify general christian insight without
counterquestions, without metaphores, without implicit assumptions,
without solipsism and without twisting of meanings.
While I'm at it, however, I'll point out that the underlying assumptions in the question are that:
- God isn't real
- The existence of Hell is immoral
In other words, the belief that we need to justify this is based on the assumption that God is a man-made construct. That we, as the group who believes in this made-up being, are responsible for introducing the idea of eternal torture. And that we must somehow answer for it or justify it. That is, in itself an "implicit assumption". It is quite impossible to have an opinion on anything without implicit assumptions. We all have them. We all have a set of glasses through which we see the world, and a base set of implicit assumptions that color our perceptions.
The very idea that "God is not real" generally springs the idea that "I won't believe it if it's not proven to me". In other words, if you can't prove that god exists, to my own satisfaction, then I don't believe He does. Tracing this back further, the root assumption that leads to this is that you are trusting only in your own intellect, mind, and reasoning process. This is the very solipsism you sought to ban in any "satisfying answer".
To bar implicit assumptions and solipsism in an answer to a question that is dripping with them shows a bit of a double standard. But at least I think I avoided the metaphor.
Likely, however, this won't be a "satisfying answer". If you were hoping for someone to come with proof that Hell is justified, I'm sorry, this is the wrong site. We don't focus on what's true, but rather what's taught. You may wish to go to a site dedicated to such things, but this isn't it.