Why does Maurice feel guilty when he kicks sand into Percival's eyes?

You can find the answer to this fairly early in Chapter
4.  It is about 4 pages in to the chapter.

The reason that
Maurice feels guilty about this is that he has some remaning memories of civilization
and what civilized people are supposed to act like.  He remembers that he once got in
trouble for doing the same thing to a younger kid's eyes back in
England.

So it is a memory of what the rules are that make
Maurice guilty.  This shows that he still has a conscience and has not become a real
savage yet.