What do you think Ralph has learned about human nature by the end of the novel?

The very ending of the story puts your question into context. Ralph learns that man (boys) gravitates, with a few exceptions, towards darkness and evil. Golding ends this story with a "deus ex machina ending". That is "the God in the machine" abrupt conclusion to the story. Ralph has pushed all his anxieties deep inside of him just trying to be a good leader and then simply survive himself. This is the first time Ralph can begin putting all the hellish events in some kind of context. He knows his childhood has been lost, his innocence destroyed by the emotional and physical violence of power and politics. Ralph realizes just how dark mans' heart is. Given a choice between good and evil, man will naturally gravitate to evil. Ralph finally realizes how much of a friend the fat marginalized boy Piggy was to him.