I have never read Marion Zimmer Bradley's work. Knowing what I know about her now, though, I have no interest in ever picking up anything she or her husband wrote.
Okay, so how about stuff she edited? MZB was for many years editor of a fantasy magazine that gave voice to loads of good new authors - do we boycott those works too, since she touched them? Would doing so be fair to the authors who worked with her, who had no idea what was going on at the time?
And how about this - the publisher of MZB's digital backlist is donating all income from sales of her works to Save the Children. Going forwards, sales of her ebooks are *helping* kids. The author who is continuing to write in MZB's Darkover universe is similarly donating proceeds to charity.
Hypothetical: What if the proceeds from those works were going to her estate, and her heirs included those who accuse her of abuse? Boycotting her works would then be taking financial support from those she allegedly harmed.
These things are often not simple.
Lovecraft is a very interesting case. He was racist, but it's really more accurate to say that he was broadly xenophobic, even when it came to things beyond race.
The man was also born in 1890 - he was born, lived his life, and died before what we now think of as the civil rights movement started. You have to work a bit to find authors (or even just people) from that time who *weren't* racist (or sexist) by today's standards. I think we should give thanks to those who are ahead of the curve, but it is hard to fault someone for failing to be ahead of his or her time.
There is a point where we are no longer avoiding support of a repugnant person's ideals, and stepping into whitewashing history - in the "those who do not study history are doomed to repeat it" sense. What's better - to avoid the work and forget, or read it and remember?
We can then also apply that thought to Card. What's better - to not support his work, or to read it and use it as a teaching/learning opportunity?