Therein lies the problem with your entire argument here. You are judging people, rather than their actions. This is a critical distinction.
Consider, for example, John Newton. He was a captain of not just one, but several slave ships, enslaving, murdering and raping who knows how many hundreds of human beings. But then he had an epiphany, which was the basis for his poem Amazing Grace, and went on to become one of the most important figures in the British abolition movement. According to your argument, he was an "awful human being" with "No exceptions." So should we stop singing Amazing Grace because it was written by an "awful" slave trader?
Or consider Moses of the Bible. He was raised as a prince of Egypt after all. He lived a life of luxury because of the labor of slaves. But then he chose a different course, and led one of the biggest slave uprisings in history. Was he an "awful person" because he once sat atop a system of slavery? Should we "cancel" the Ten Commandments he brought down from Mount Sinai?
Your argument results in even worse consequences when applied to living people. Calling someone an "awful human being" a "racist" or a "bad person" means that this is who they are - it cannot be changed.
We should deal with all people as we (should) deal with our children. Don't call them a "bad" child when they do something wrong. Rather, point out the action they did, and explain why it was wrong. Then they can change, as John Newton did. Once you condemn the person, however, you have given up all hope that they can change for the better.
As for your title and overall thesis - Yes, we should apply today's moral standards to past behavior - I think you are missing the point as well. The point is not which set of moral standards to apply, but how to apply them to a different time and place. Slavery is always evil, but what do you do when you are in the middle of it, whether as the one owning or being owned?
Before you answer that question, I recommend you read Octavia Butler's novel Kindred. She wrote it in reaction to something a friend of hers said - someone she looked up to as knowledgeable about Black History. "I wish I could kill all the Black people who are holding us back" (by going along with oppression, e.g., the "Uncle Tom's); "but" he qualified, "then I would have to start with my own parents." Butler thought of her Mother, who put up with all sorts of abuse working as a maid for rich White people. Her Mother put up with that abuse, without a word of protest, in order to give her daughter a chance for a better life. So what would a modern Black woman do if she was sent back to the time of slavery in the South? That is what Butler explored in her novel, throwing her protagonist's White husband back in time as well. What would you do in that situation, really?
This is something I have given much thought to lately. Watching the storming of the Capitol, I could not help thinking of the Burning of the Reichstag and the rise to power of the Nazis. I could not help thinking of my own relatives (the few who survived) who fought back against all odds, including one leader of the Warsaw Ghetto Uprising. Could I do the same?
I don't know.
What I do know is that placing blame on people, rather than their actions, just makes things worse. None of us is qualified to throw the first stone. We all have both oppressors and oppressed in our ancestry. All of us need to think carefully about what we can and should do in the present circumstances we find ourselves to make this a better society.