It was a Holocaust - millions died - and you did deny it ("didn't intentionally set out to kill people...a bad system...history is quite complicated"). Zero hyperbole involved. Exactly the same weasely language as you'd find with respect to WW2. What you said here was wrong, factually and ethically.

The people Mao murdered should indeed be remembered over Mao himself, and mentioned whenever his name is mentioned. The deaths were denied at the time and they are all-too-frequently elided now. We need to remember them.

while we're at it, no one should ever be allowed to mention any pre-Civil War leader without a discourse on slavery, or Columbus without mentioning his murders and barbaric practices. There are enough historical atrocities to go around, no one should have to risk having a discussion without their personal one being overlooked.

You didn't just say that it was not relevant (wrong as that might be). You denied intent, denied that it was purposeful.