> Considering that his rhetoric was very much based on Christianity, it's clear what standard of "unjust" he was applying.

Considering the diversity of standards of justice within the history of Christianity (which, in just the US, includes—relevant to this topic—MLK, sure, but also the Southern Baptist Convention, founded explicitly in support of slavery), I don't know that having rhetoric grounded in Christian theology tells much of substance about the standard of justice one is appealing to.