Complex sentence: Duelling had been outlawed in France and this could be considered a transgression against hospitality, in addition to the fact that William Crofts was a powerful figure as the Queen's Master of Horse and head of her lifeguard.
Split into simpler sentences: Duelling had been outlawed in France and this could be considered a transgression against hospitality. Besides, Will Crofts was the Queen's Master of Horse and head of her lifeguard.