Question

what probably motivates Mrs Hamilton to behave in such a manner toward the slaves

Answer 1

if Mrs Hamilton is upper class she would see the slaves as peasants in a way. as I do not know what the slaves may look like, if they are black you could talk about racism and the fact that in the 1950s segregation was a huge issue. blacks were used to do chores for the whites. 
hope this helps in some way 

Answer 2

I also think that Mrs. Hamilton's cruelty comes from the behavior of her husband.