I didn't think he was going for "realism", although he did write it in response to a children's adventure story called The Coral Island. You're not wrong at all to disagree with the themes running through the book if that's your take away from it, but I didn't see anywhere that Golding was saying that human nature was inherently callous and bloodthirsty without organised society, I thought he was just saying that those feelings are inside us. In a way they hardly do better than the adult "civilised" society, where there's apparently a big war going on (Golding served in WWII, which really disillusioned him as to his belief of the goodness in humanity).