Here is a very initial thought:
In answering this question it is helpful to know what torture actually is? Let's say for simplicity that torture involves
- the deliberate infliction
- of extreme forms of mental or physical pain
- in order to achieve some goal.
The goal itself may be legitimate (e.g., the extraction of critical information from a suspected terrorist) but the means is considered not to be.
Is Hell a torture cell? Well let's see
Does it involve extreme forms of mental or physical pain? It appears so - a painless Hell seems unlike any Hell I have heard of.
Is the pain used to achieve a goal? So it seems. And what is God's goal in sending people to Hell?
(a) to punish people because they deserve it?
(b) to purify them?
(c) to reform behaviour?
(d) to educate people on the true reality of sin?
Perhaps all of those things and more - must God have only one purpose? And these purposes are all good ones but, of course, that is not normally considered a legitimate defence for torture.
So God is using extreme pain to achieve a good goal. Does this not make God a torturer?
That depends. If God deliberately inflicts the pain from the outside of the person with the intention of achieving a good goal then the answer may be "Yes". Of course, even in this case, there is a big disanology with torture as normally understood. The pain is intended - in (b)-(d) above, at any rate - for the good of the person suffering it and for no other end. God is seeking the ultimate blessing of the one in Hell. The torturer is not doing that. Even so we might still remain very uneasy with God's extreme method.
But Hell need not be understood in that way. Hell could be understood not as the infliction of pain by God but as a condition in which God allows a person to experience the inherent consequences of their sin. Sin is a corrosive element in human life and God shields us in this age from its full impact. Hell could be seen as a place where God stops shielding us and allows the corrsive power of sin to take its course. This would be a very horrible experience that could serve goals (a) to (d) above but in this case it does not seem that God is torturing us at all. He allows the pains of Hell and uses them to achieve his goals but he does not inflict them. He is not torturing us but leaving us to experience the consequences of our own actions.
Such are my hasty initital thoughts. I am not sure about them yet but you're welcome to do with them as you wish.