CS Lewis, let's analyze this quote shall we?
"My argument against God was that the universe seemed so cruel and unjust."
The universe is cruel? What's so cruel about the andromeda galaxy? Sorry CS, I don't see a lack of justice in solar systems, stars and planets.
"But how had I got this idea of just and unjust?"
From understanding the concept of harm, and how humans, being social animals, prefer happiness to harm.
"A man does not call a line crooked unless he has some idea of a straight line."
Sorry CS, distinguishing shapes on piece of paper and children dying every day does not correspond.
"What was I comparing this universe with when I called it unjust?"
Good question CS, you can't compare universes, this is the only one you know.
"If the whole show was bad and senseless from A to Z, so to speak, why did I, who was supposed to be part of the show, find myself in such a violent reaction against it?"
The whole show is bad and senseless? No, not really just the bad parts, like religious faith and dogma, or human slavery and genocide. Sorry you had a violet reaction, I can't relate to that, maybe you need pscyhological help.
"Of course I could have given up my idea of justice by saying it was nothing but a private idea of my own. But if i did that, then my argument against God collapsed too--for the argument depended on saying the world was really unjust, not simply that it did not happen to please my fancies."
No, you can't give it up, you are a social animal, sorry. You could say that justice is a private idea of your own, but it's not, never was, never will be. There could be no murder or rape or war for the next 2000 years, we could live in a utopia, and there would still be no evidence for God CS. Besides the existence of evil is just one of the many pieces of EVIDENCE that God doesn't exist, not an argument that he doesn't exist.
"Thus, in the very act of trying to prove that God did not exist - in other words, that the whole of reality was senseless - I found I was forced to assume that one part of reality - namely my idea of justice - was full of sense."
Your point is that if God exists reality makes sense? How so? I don't see the connection here AT ALL. Your idea of justice was full of sense? Well I'm not sure what the hell your idea of justice is. A Bible that sanctions slavery?
"If the whole universe has no meaning, we should never have found out that it has no meaning: just as, if there were no light in the universe and therefore no creatures with eyes, we should never have known it was dark. Dark would be without meaning."
Stars live, stars die, and so do their solar systems, is that a meaningless cycle? Does your diety's existence add meaning to this cycle somehow? When a tree falls in a forest, it still makes a sound, whether someone hears it or not.