When Stalin died plenty of people were too scared or shocked (in his Empire) or too happy (elsewhere) to bother to comment.
The taboo might have something to do with allowing the family to grieve, and people do come off looking obnoxious if they make gleeful comments about someone's passing on the day they die.
There may also be a notion of reciprocity behind it. One talks well or not at all of others when they die in hopes they'll do the same when one passes on oneself. In today's Internet world, though, there's always somebody somewhere who breaks the taboo.
I guess there's a threshold between people who are really unredeemable, like Hitler or Stalin, and those who might have some saving feature. In Sontag's case, she did speak out against Communism in 1982, too late perhaps but with a force that implied a condemnation of her earlier enthusiasms, and she did introduce some important writers to the American public. Is that enough?
However obnoxious she was in her prime, she looks more and more like a figure out of the distant past. Some people will feel a need to remember, others don't or would rather forget. I'd just as soon forget her offenses -- and her with them.
Sontag was more than an ordinary novelist whose politics are beside the point, but less than one of the world's actual decision-makers. So how one responds is a judgment call, and people will disagree. I wouldn't have any problem keeping quiet for a few days, if nothing else it would give us a chance to think over what we really want to say.
... a beautiful eulogy.