In Chapter 12 of Lord of the Flies, why does Golding use the second person point of view?
Great question! I think it allows Golding to freely show us the thoughts pounding through Ralph's distracted brain. We've seen him get more and more cloudy in his own head (that famous quote about the "bat's wing" which obscures his clarity of thought in an earlier chapter) and now we actually move the camera inside his head. He's on his own.
The seconds lengthened. Ralph was looking straight into the savage’s eyes.
You’ll get back.
Now he’s seen you. He’s making sure. A stick sharpened.
So in this quote, firstly we have third person ("Ralph looked..."). And then we get Ralph talking to himself: "Don't scream". Then we get, what is actually a quote from Simon earlier in the novel, playing Ralph's mind: "You'll get back". The second person has the effect of creating Ralph, in his head, talking to himself, using Simon's words, but also using his own ("Now he's seen you...") and so on.
I also think there is a tactical reason for pulling the camera towards Ralph's perspective. It prepares the ground for that fascinating perspectival moment when we see up the naval officer's body from Ralph's perspective:
He staggered to his feet, tensed for more terrors, and looked up at a huge peaked cap. It was a white-topped cap, and above the green shade of the peak was a crown, an anchor, gold foliage. He saw white drill, epaulettes, a revolver, a row of gilt buttons down the front of a uniform. A naval officer stood on the sand...