Piggy is one of the most sympathetic characters in the book, the voice of intelligence and reason, shown in the way that he conceives the idea of the assembly heralded by the blowing of the conch. This is the mark of democracy, of civilized debate and order. Piggy's violent death at the hands of the brutal Roger symbolizes the final destruction of reason on the island as all-out savagery and anarchy ensue.
However, although Piggy has sterling good qualities, he also has his flaws. In being notably overweight and physically inactive, he does little to help the other boys with the hard physical work they have to do on the island to survive. He is the one with ideas, but in practical terms he is not very efficient. For instance, at the beginning when the boys are beginning to gather, he realises the need for a census but doesn't know how to carry it out.
A more serious flaw is that Piggy tends to criticise others too much. This can be seen in the way that he often dismisses the other boys - even the bigger boys - as mere 'kids'. This makes the other boys resentful and more likely to retaliate against him. He is intelligent but he is not tactful - one of the reasons why he is not leadership material.
A third flaw, and certainly the most tragic, is that Piggy relies on his own beliefs and principles too much. Endowed with a superior intellect (certainly when compared to the other boys in this story) he naively believes in the triumph of rationalism, and cannot, or refuses to, accept that reason and order cannot always prevail, even as he witnesses the descent into anarchy all around him. This is most strikingly apparent just moments before his death, when he and Ralph are under attack from Jack's group, and he still insists on holding the conch while demanding:
Which is better—to have rules and agree, or to hunt and kill? (chapter 11)
To the last, then, Piggy appeals to reason and order, to civilised values, only to have his life ended by an act of irrational savagery.