COVID? Seriously? Is this a 5th grader? It doesn't even snag the top 100. Maybe handling of disease outbreaks as a whole is top 5. But those aren't caused by humans so much as they are a natural part of living in an ecosystem.
Slavery has been going on for 5000 years, and still does, including in many forms not called slavery. There will always be the haves and have not. In many cases slavery, serfdom and similar were a function of society to keep those without anything from essentially dying. I mean, those with resources could easily kill their captured prisoners or beggars or the weak or the oppressed. It's definitely a top 5 though when it is done in a ruthless manner with no benefit at all to the oppressed people.
War in general is by far the #1. Whether WWII, Ghengis Khan, Pelopnesian Wars, tribal wars, and on and on and on since the beginning of time. It will never stop. Ever. If I remember there have been 4 years since year 0 where there were no wars of some type. People will always protect what is theirs, and when necessary take what belongs to others. War may be a fact of life, but it's held humanity back immeasurable amounts. The sheer dollars the US spends on the military, if able to be devoted elsewhere, would have done so much to advance humanity. And this comes from a hard core conservative.