Why is Ralph unfriendly towards Piggy in "Lord of the Flies"?
He can't help himself, really. In the same way that he joins in with the hunts despite himself ("the desire to squeeze and hurt was over-mastering") so too he can't quite resist that inbuilt impulse to make fun of the bullied kid.
'He's not Fatty,' cried Ralph, 'his real name's Piggy!' 'Piggy!' 'Piggy!' 'Oh, Piggy!'
A storm of laughter arose and even the tiniest child joined in. For the moment the boys were a closed circuit of sympathy with Piggy outside: he went very pink, bowed his head and cleaned his glasses again.
This example, right at the start of the book, shows Ralph's unwilling (and, perhaps unmalicious) urge to mock Piggy just as the other boys do. Despite Piggy's good qualities (clear-sightedness, wisdom, good counsel) he is still mocked because he is fat, asthmatic and wears glasses.
It's a good question - and the answer actually points to Golding's central thesis: that all human beings have something nasty at their very heart. "The darkness of man's heart", as Ralph weeps for it at the very end of the book's final chapter.