Yeah, I think we're broadly agreeing, I just think we need to be extraordinarily careful about how we implicate society when talking about individual behaviour. Maybe you agree with this too, I'm not quite sure.
I mean, yes, those three men wouldn't have killed Emmett without the backdrop of a deeply racist society. So society is relevant. And even in examples that are less grotesque than that, society's influence is still relevant. America's issues with race, for example, are pretty much unique in the western world, and that uniqueness comes from the unique components of American society.
But making up American society, are countless white people (I believe the majority) who would never dream of mistreating a black person on the basis of their race like Chauvin or Zimmerman or McMichaels did. There are black people who kill other black people with no regard to their shared race. There are black people using their newfound power in society to discriminate against white people. And of course there are white people being racist to black people just as there always have been in America. All of these different dynamics exist simultaneously within the same society because "society" isn't useful for predicting individual behaviour.
So, going back to Emmett's murder. If you point to the culture of racism in America at the time, there's no denying it's a factor. If you want to blame the legal system's protection of white people when committing crimes against black people, that's definitely a factor too. But surely the actual blame must lie with these three men. And certainly can't lay with the millions of other white people who were nothing to do with it, no?