The weather on the island grows increasingly more hostile and ominous when Simon kills. But why?
It's called pathetic fallacy. It's when a writer uses elements of the natural world (weather, nature, animals, and so on) to reflect what happens in the book.
Golding uses the weather and the island to suggest or foreshadow events in the novel throughout. So, right at the start, we learn that the plane ominously was shot down during a storm:
Some act of God— a typhoon perhaps, or the storm that had accompanied his own arrival—had banked sand inside the lagoon...
And, after Simon's death, as the storm clears, there's an unusual section at the end of the chapter in which the weather clears and nature seems to almost reabsorb Simon: in the sky, the anger/aggression of JAck becomes Simon's relaxed sensitivity:
Towards midnight the rain ceased and the clouds drifted away, so that the sky was scattered once more with the incredible lamps of stars.
One last example. On the first page, Golding includes an ominous little image which gives the impression that the island might turn out to be negative, though it seems like paradise:
He was clambering heavily among the creepers and broken trunks when a bird, a vision of red and yellow, flashed upwards with a witch-like cry...
"A witch-like cry". Read the paragraph again, and feel the spookiness of that image. Golding wants you to be spooked - but does it subtly through pathetic fallacy.