There is a huge difference between Acting polite and actually Being polite.
Is there?
What is politeness other than a matter of how you act?
I'm pretty sure there have been some very brutal dictators who were charming in person.
Indeed, it's easy for those with the most power to act 'nice' in person, precisely because they can have others carry out their crimes for them. I don't for a moment think that should be used to excuse them.
Trump may be unusual in being a horrible guy both politically at-a-distance and up-close-in-person. But that certainly doesn't excuse the Chinese regime, or Xi himself.
(It would be a very strange alternative universe where Xi and Trump were swapped over, come to think of it)