Simple. How humanity responds under stress and how quick they are to return to barbaric ways.With the exception of Michael (who gives us an outsiders view of humanity but in a way that is in no way universally relatable), I didn’t feel that they really did explore humanity that much in DSC.
But my brow was heavily furrowed whilst I was watching it so I may well have missed something - how else did they explore the human condition? (I mean that sincerely as well - not in a confrontational way)
Now, I know there will be contention that Star Trek is about optimism and about humanity becoming better and I'm all for that. So, perhaps DSC would have been better served to demonstrate that optimism and then challenge it in another season. But, that's not what we got so I work with what's there.
For me, it is the idea that the fear drives humans and others to do irrational things. It makes us consider options that we might otherwise not engage, to behave irrationally even if we don't know why.
As much as I know people want Star Trek to be optimistic about humanity, and perhaps see a more evolved sentiment, this is an era in which bigotry still exists, where humanity is still facing down the challenges of hostile powers. I hardly expect perfection.
It also reminds me of this quote from "Red vs. Blue" (A Halo machinima):
Have you forgotten sir, we were at war? A fight with an alien race for the very survival of our species. I feel I must remind you that it is an undeniable, and may I say a fundamental quality of man, that when faced with extinction, every alternative is preferable.