What was Edmund thinking?
Edmund of Cornwall has got it made; King Arthur's right hand man, Katherine's husband-to-be, plenty of land, handy with a lance, nothing to do but boss the serfs around. And what does he do? Sell out Arthur to the Saxons so he can be king and inherit all the headaches Arthur has as king.
Reminds me of the old Republic serials where American thugs hire out to aliens (Purple Monster Strikes) or Japanese spies (Masked Marvel). What exactly do these thugs think is going to happen to them if the aliens/Japanese are successful and take over the USA? Well, it happens to Edmund!