"People are mean."
Note: 비열하다 might be the best literal translation, but is not very often used in colloquial language. It's more common to just say they are bad, or 나쁘다 (나쁩니다).
In the other lesson they used 사람은 for people in plural, is it the same thing?
I've read that the plural marker 들 is optional. You can use it to emphasize the fact that it is plural, but you can also leave it out. (But OMG, I know so little -- so please wait for others' comments. ...)
Oh, I actually seem to be right; there's a similar discussion: https://www.duolingo.com/comment/24421391
들 - Is use to emphasize that the noun is plural
If a noun does not have 들 , in can mean singular or plural .
If you want to emphasize that it is singular , for example , There are just 1 Person ' , You can use : 한 (One) 명 ( Use to indicate number of people ) 사람 ( Person/People )
한 명 사람 있어