Like I said, it's because he KNOWS humans really aren't that bad compared to his own people yet the way he talks he acts like they are.
When he tells McCoy that humans have a tendency to not care when millions of people die vs when individuals die (the "Immunity Syndrome" I think) was rather unnecessary and I don't get why he felt the need to take a swipe at him in that situation. And heck, the Ancient Vulcans probably slaughtered like that all the time and similarly didn't care.
And what about the Klingons and Romulans, they probably kill a lot more yet he never brings them up.
Honestly, he should know better than anyone on the ship that just because they act emotional NOW it doesn't mean they'll always be "barbaric" (because compared to the other aliens they meet, they aren't) and if the Vulcans (who were worse than humans) could get better than there's little point in dissing humans.