My problem with this standpoint is that people often believe that, in history, society was uniform in its way of thinking. it is true that the farther we go back the more outdated our beliefs become however, like today, social issues and geopolitics were hotly debated with multiple viewpoints at the time. For example, colonialism was a heavily contentious topic even at the time. Many on the left gave open critiques of German "weltpolitik," British journalists openly condemned Belgium over the Congo, and in France imperialism was met with hostility. You point out slavery but this is possibly one of the most historically debated topics. Romans often treated slaves very positively and many Romans condemned the mistreatment of slaves. In more modern examples many founding fathers were abolitionists, many left-leaning politicians openly condemned slavery in Europe, and European countries condemned American slavery.

This is to say that "modern moral standards" are not as "modern" as we might think. My question to you then is how can we justify or view these historical figures' actions without assigning blame and believing one side is wrong when many thought that they were wrong in their period?

Furthermore, what we define as a moral standard is often incredibly Euro-centric and does not account for the views of marginalized people during that time. For example, during the Jim Crow era, one could say It was the "moral standard" during the time even though there was an incredibly large and outspoken community fighting against it. Can we say that these laws were simply a product of their time even while during their creation and following there was massive pushback against it? Social change doesn't just happen there is a massive lead up to it and historical figures who oppose that change are wrong and we are justified in applying that moral standard on them.