Many of said historical events and persons were immoral with the morals of back then as well. Murder has always been bad. Slavery has always been immoral. Genocide has always been bad, but guess what happened when the Spanish invaded the Americas?
A position does not become objective by being the consensus opinion.
I value human life. If another human values human death, there isn't a sense in which my value is necessarily more objective than theirs.
A summary of the CI, for those unfamiliar with Kant:
Suppose you'd like to do something. Could you wish that everyone everywhere all the time behaved that way? That is, consider the hypothetical world where everyone everywhere did that thing as often as they wanted, and see whether you like the outcome (or, would prefer that hypothetical world to the one where no one anywhere ever did that thing).
If the world associated with the action you're considering is undesirable from a rational standpoint, then it's literally insane and a wholly despicable thing for you to do it. This is because, if you do it yourself while wishing no one else to do so, then you are considering your own self (or your own family or your own tribe) to be an exception to your own rational assessment of that behavior if everyone else were to do it. There is no rational basis for considering yourself such an exception just because you are yourself. So, don't do that thing.
Kant goes to extreme lengths to show that the Categorical Imperative is not just a nice idea that he dreamed up, but is rather an intrinsic entailment of rationality itself.
So, murder is objectively wrong because the idea of everyone, everywhere, in every time doing it would result in a horrible world that no rational person would prefer to the alternative where no one anywhere ever committed murder. Same with slavery and genocide. They're evil because if everyone everywhere did it, the resulting world would, if examined rationally, be worse than the world where no one anywhere ever did them.
We cannot all do the same things. For example, there are things that adults do that are necessary for the survival of humanity that children should not do.
So your response would be to add more provisions, yet you can always add more provisions, because there are many ways that we cannot be the same. Here's a trivial one: if your name is Bob, you can kill people, otherwise it's wrong. You can replace that with all sorts of reasons why you are justified and others are not. You are not constrained in how you frame it. It's easy to use murder as an example, because we already intersubjectively agree it is wrong (murder is also wrong by definition).
Let's get to an example closer to home for many people: I think it is clear that, using the categorical imperative, not having children is morally wrong because it would end horribly for the remaining aging population. I doubt you'd agree with that. You can only argue that the predicted result is not objective, but which predicted results are?
Also, if I see someone doing something which I don't think is viable on a larger scale, am I to conclude that they're evil, or is the categorical imperative only meant to be applied personally?
The mass murder, slavery, and imperialism of the Aztecs was replaced by the mass murder, slavery, and imperialism of the Spaniards. But who, after all, speaks today of the annihilation of the Otzoma?
The issue isn't whether we apply ethics from a relativist or objective standpoint, but that we pick and choose when we do. Right now, Europeans must always be judged by contemporary standards (ironically rooted in the Enlightenment), while other cultures must never be judged.
They put a stop to the practice of human sacrifice, which was rampant throughout the Aztec Empire?