Why does Ralph chastise the boys during the assembly at the beginning of Chapter 5?
He has lots of good reasons to chastise the other boys. In short, it's because they are getting lazy, and the civilised rules that were established for the good of everyone are starting to break down!
Firstly, people are no longer leaving fresh water in the appointed place: (this is Ralph speaking)
“...we said we’d have the water brought. And now not. There were only two full shells there this afternoon.”
Moreover, people gradually stopped helping to build the shelters, meaning that there are two strong shelters and one weak one (built alone by Ralph and Simon):
"...me ’n Simon built the last one over there. That’s why it’s so tottery. No. Don’t laugh. That shelter might fall down if the rain comes back. We’ll need those shelters then.”
People are also not using the appointed lavatories:
We chose those rocks right along beyond the bathing pool as a lavatory... Now people seem to use anywhere. Even near the shelters and the platform.
Ralph also talks about the fact that people keep letting the signal fire go out, and the way that little fires might cause a bush fire: fire, he says, will only be allowed now on the mountain.
Why, does he chastise them? There are lots of details, but, in short, because...
“Things are breaking up. I don’t understand why. We began well; we were happy. And then—”
The beast of savagery is closing in....