why do you think her dad hated black people so much ?

Ruth's father was a racist, which was probably borne from the prejudice he dealt with all of his life as a Jew. He shouldn't have taken advantage of black customers, and he shouldn't have acted the way he did, but I think his reaction to and treatment of blacks had everything to do with the anger he felt at how he himself (and all Jews) were treated. General opinions of blacks at the time would have given him the opportunity to act inappropriately.