There are lots of things negative about the world, humanity, and society in general. People aren't always nice, decent, enlightened, and/or considerate. Awareness is relative - you don't know what you don't know. Sometimes you learn things you wish you didn't. It is to this which I am speaking. Sorry if it isn't the "light" kind of uncomfortable you can laugh about later. I'm not talking about walking in on your parents.
What I will bring up is slavery, cannibalism, and abuses based on race, gender, age, religion, education, and class - to name a few. Apparently throughout human history the notion of "playing nice with others" struggles to stick. Social divisions come with society, it's that simple.And it's typically not viewed in favorable light...except perhaps by those gaining from the division and oppression.
A brief reminder before I drag us down even further; in my studies of anthropology, sociology, and history I have been admonished by professors to not judge the past based on the present.
Consider this, nearly every human society this world has known has some instance of slavery, cannibalism, and oppression. Before you get all defensive about it and start throwing out "nu-uh, not mine!", really stop and think. Did everyone always get along? Did everyone always get their fair share? Did everyone always have enough to eat? No one ever made someone work for them? Yeah, no. I didn't think so.
So if you're building a world or writing a story, think about these atrocities. There is certainly a lot to be learned from them. Are there situations of folks nom-noming on each other? Keep in mind that such grizzly business usually only comes up in extreme scenarios. For reasons of simple growth and attrition, it can't be kept up indefinitely or you'll run out of neighbors to crunch and munch. Is there/was there a slave class? Who were the masters, and who/what were the slaves? Were they people or machines or animals? What was the work? If there was emancipation/revolution, how long ago? If it's in process, how/who/what started it? If slavery has been completely squashed, how is it enforced? How do former slaves and slaveholders feel about it? What was the slavery based on? Are the former slaves shunned, driven out of previous areas of servitude, or is there outright violence against them? To what degree, beatings, rape, disfigurement, murder?