You would need to be very careful when defining "crimes against humanity" to not be post-facto law. It is like the maxim of it being better to let a hundred guilty go free than punish one innocent: if being innocent is no defense then why would anyone bother to be innocent?
What the Nazis did was totally legal in the Third Reich, back when they did it. They made sure of that. Yet it was totally clear just how against the humanity it was afterwards. Should the victors have let them go at the Nuremberg trials? No, so they used international law and saw this as a starting point for the universal declaration of human rights.
I think it is good that people and companies should think about what will be seen as a crime against humanity afterwards. I'd say anything that goes against the universal declaration of human rights now is a good start, even if there are national loopholes right now. Actors that know now that they are e.g. poisioning the environment/people or actively prevent a solution to climate crisis with millions of starving people on the other side of the scale, know today that what they do is wrong. They just think they will get away with it.
The question is about spirit of the law VS letter of the law. Just because all you did was legal doesn't mean you are innocent or right. Just because you did something illegal doesn't mean you are wrong (e.g. it was illegal to hide a jew in the Third Reich).
So some of the things individual actors so is already illegal under international law and/or a violation of human rights. And I think we would all profit from the faint possibility of that ever becoming a thing.
