we don't know that her parents knew the guy was a sadist. it's the early thirties. PSAs about wretched men were far and few between. A monstrous human being really, honestly, is out of the imaginative reach of most people. They expect to see them glimmering on television, with an ominous soundtrack.
A regular guy? Remember the comedian's bit- it's always the quiet guy? that's why policeman make hard neighbors- they can sense the darkness, and they sound a little nuts, when they are describing the little linty odd bits of behavior to other people. Not even writers who make a living at this, really, are much good at describing evil, usually. I've read police books where they say they go to writers' guild meetings, and the writers are horrified- the policeman is describing them, or their neighbors, and then describing scenes of horrifying desecrations of human bodies, usually far worse than what the writers had imagined. They say, usually, that the writers---mystery writers---people making a living imagining a killer doing terrible things- go lose their lunch b/c they literally cannot stomach the reality they are trying to approximate.
So, while her parents are foolish when presented with evidence that we, ourselves, would know as danger signals- they do have the claim of innocence of men's dark hearts.
I think that is what fuels Rosalie's ferocious honesty. It's hard to speak little lies, when big lies have cost you everything dear.