In the movie, Ennis was actually satisfied with eating beans because he was not much of a cook. But, as to how much Annie Proulx's Ennis Del Mar liked beans, that would have to be a guess. (There is one kind of beans which I don't like and they are called "Blackeyed Peas." I tolerated them growing up when we were so poor that we had to eat them.)
Joe Aguirre would not have been able to prove that they had slaughtered/butchered a sheep if he didn't see them do it. And, it would have saved ammunition if they killed the sheep using a knife.
