Was the big melee at the end necessary? I mean, the position the English had against the French was far too advantageous to sacrifice, considering the French's numbers advantage. Why didn't the English just stay at the top of the cliffs and just rain arrows the whole time?
Because Prince John wanted to lead the charge, because all it would take is the French to start getting organized and getting some people up past the beach to start coming in from the archers' flank, and because war is not always fought in ways that seem rational to someone watching from the vantage point of a movie camera.
Why did Godfrey betray King John? He was already the second most powerful man in England. What could the King of France offer him to beat that?
Being the most powerful man in England as well as the second most powerful man in France.