A lot of these motives have to do with the fact that William was a nasty piece of work. Before receiving his knighthood, he was a very successful factory owner who made his fortune during the war. Over the course of the film, it comes to light that he used many of the girls who worked in his factories for sex. His elevation in society did not change his proclivities; at the time of his murder, he is carrying on with Elsie, the head housemaid at Gosford Park. Also, William antagonizes many of his blue blooded but cash poor relations by marriage by wielding his wealth like a cudgel... they hate him but need him.
So that's an introduction to the main upstairs cast cast of Gosford Park. Next time, I'll concentrate on the denizens of below stairs and what roles they play in the occurrences above stairs.