I think you have to criticize individuals within the context of the society they've been living in. That doesn't mean that anti-semitism was "good" in the 19th century, of course not, but I do think that it's worse to be an anti-semit now than before. That is the difference, imho. Bad personality traits are still bad, but when society has reached a point where a lesson should've been learned once and for all (e.g. Holocaust in this case) or society has changed significantly (people used to grow up in entirely anti-semit environments, which isn't the case anymore in Germany, at least) and some individuals still don't change and/or inhabit these traits, then the bad personality trait reaches another level, imho. It's a difference in quantity, not quality. Choosing to be bad is worse than being taught to be bad from the get go is probably the essence of my point. Granted, that still doesn't mean I approve of people being anti-semits at any point in history, since there were people who weren't anti-semits. One has to alter his perspective a bit, at least, though. This was a *huge* topic in German literature during the war and it split the literary world into three worlds, because there were authors who ignored the war and everything about it completely and continued writing about other things, some authors who approved of it and some exiled authors who criticized it immensely. I am not sure what to think of some of the authors at that time, especially the ones who ignored it. I do think a serious artists reflects on current society and should be contemporary and pushing the limits to a degree.
Someone being an "asshole" (in the sense of being rude to people) is not enough for me. This woman was more than that, though. I think I'm personally also a bit sensitive to people being disrespectful to the suffering and pain caused by the second world war by legitimizing mass murder with it, now. Sadly, many people are abusing it to this day. The context of her words is particularly troubling, to me. It saddens me deeply that a Jewish woman would say these things. I also don't think it makes sense to compare Hemingway to Newton, if we limit Newton to the works he wrote about mathematics and physics, which are the relevant ones, to me. So in that case it's artist vs. scientist and I don't treat those similarly.
I don't think "society" can be defined in strokes that broad. There have always been plenty of separate "societies" with different moral values even within the same countries simultaneously. Not to mention the fact that that says nothing about the individual's upbringing.
History is also littered with "upsetting events". Being racist was still apparently a-ok in the books of several late 19th and early 20th century authors, even though the Emancipation Proclamation had been signed decades before (and earlier still in many other countries), putting an end to one of the most vile trends in the history of our species. Didn't stop many well-regarded thinkers from thinking of blacks as inferior though.
I still stand by the fact that I would read Rand's books if they were good, independently of the fact that she's an utter pillock. I know nothing of her upbringing and of her experiences with society, and of whether she was conditioned from an early age to think like this (keep in mind this does not excuse her in the least, and is merely a hypothetical situation). Conversely, many authors in "old societies" could have been brought up in caring, open-minded families and then simply "fallen back" into retrograde views, and this I find far more aggravating.
As for Newton, he wrote several very well-regarded essays. He also liked to directly ruin the lives of his contemporary colleagues for the sole benefit of his own ego. Say what you will about Rand (and there is plenty to say) but I doubt she ever directly laid waste to a fellow man's legacy.
Long story short; I can totally see where you're coming from, but if Ayn Rand had written something on the level of "Light in August", I'd eat that shit right up. I concretely know next to nothing about the lives and views of many authors I admire.