But who are you to decide which opinions are valid and which are not?
The idea of enslaving a people is totally unacceptable by TODAY'S societal norms. But in past human history, it was standard practice for a conqueror to enslave the conquered. By today's standards - that is evil. By the standards of the day, it was an absolutely normal concept.
That's part of the problem with looking back and 'interpreting' history. LOTS of things done in the past are reprehensible to modern society, but to really understand what was going on you need to apply the morals of the day, NOT modern morals. There are things we do that we consider proper civilized behavior that past cultures would consider absolutely reprehensible. Problem is, until someone invents a fully functional flux capacitor, we can only look back at recorded history, they can't see forward, nor can we, so see what 'normal' will be 1000 years from now.
No, everyone is entitled to whatever opinion they want, no matter how reprehensible it may seem to others. ACTING on such opinions is a different story. There is not nor do I hope there EVER is a "thought police". There is no crime in thinking about offing your nutcase neighbor who wakes you up every weekend by mowing his lawn at the crack of dawn, but if you actually go and attempt it - THAT is a criminal act.