This was a question I was curious about as well until I got into a debate about God’s “benevolence” with a Zoroastrian on a message forum once.
He believed his God, Ohrmuzd, was a truly benevolent God because their doctrine taught everyone makes it to Heaven in the end. Nobody has to undergo what he called, "The Christian idea of eternal punishment."
I asked this Zoroastrian, "If I have spent my entire life rejecting Ohrmuzd, essentially sending Him the message that I do not want anything to do with Him, then how is His forcing me to spend eternity in His perpetual presence a reward and a sign of benevolence?" I do not want to spend eternity in the presence of Ohrmuzd. So how shall his forcing me to do so considered Heaven?
Keeping my reply to the Zoroastrian in mind along with thinking about the predicament the God of Christianity was in when the angels, and consequently, humanity rebelled, it is no wonder He created a Hell. One-third of the angels made it clear that they didn’t want anything to do with God, outside of usurping His throne, of course. So our God, in his benevolence, essentially gave them what they were asking for. He gave them an existence where they ruled and that was completely absent from God’s presence or influence.
And of course, we call it Hell, which term also bears strongly the weight of an undesirable circumstance "Out of the belly of Hell cried I...". But, as been stated before, any existence void of God or His influence will not have a source for joy or happiness or contentment of any kind. For God is the source for all these good things. That is why Hell is associated with pain and suffering. Outside God, that’s all that can exist.
Now, as to the question of why humans who reject God must go there, there are logical answers. First, Heaven and Hell are the only two places one can spend eternity. There is no third place. There can only exist two places. A place where God is, and a place where God isn't. Second, humans who reject God are essentially telling God they do not want anything to do with Him. God is essentially giving them what they’ve spent their entire life asking Him for, that is, they’ve been asking Him for separation from Him. The fact that Hell contains pain and suffering is a corollary. It is God’s benevolence that proves to be reason people go to Hell. Yet, ironically, agnostics and non-Christians say that, according to Christian doctrine, God’s lack of benevolence is the main reason people go to Hell.
So Heaven and Hell aren't defined by the level of hostility, or euphoria of the environment. And on a subjective level, how can any place which has God's presence not be Heaven? How can any place that lacks God's presence not be Hell? I think, as Lewis did, that those that go to Heaven will find they've always been in Heaven, and those that go to Hell will have always been in Hell.
So, in concluding, God sends no one to Hell. God gives everyone exactly what they want. If you spend your life longing for God and wanting to be in His perpetual presence, then that’s what you get. If you spend it rejecting God and wanting an existence where He, or His influence, is completely and utterly absent, then that's what you get.
That’s true benevolence.