I wouldn't say he's an exception - Golding's message is that everyone has a dark side to their nature, but that doesn't mean that everyone is at the same extremes as Jack or Roger, for example. Considering Ralph and Piggy both participated in the death of Simon (and refusing to acknowledge that they did so, saying that "it was an accident"), I suppose you can argue that they're just as bad as the rest of them, but don't always openly seek to kill as some of the others do.
If you did want to argue that, however, you could definitely say that it's something to do with his natural leadership abilities and desire for "rules" and democracy from the moment the boys arrive on the island, as he may have known what has the potential to happen if there are no rules/etc. (which is ironically what ends up happening anyway).
You can argue whatever you want in Literature really, as long as it somewhat makes sense and you can back it up :>