The third book of the Hunger Games series is quite dark. I personally (as an adult) was so dismayed by the ending of the third book that I won't go see the movies. My kids are teens, so no issues with them reading them, but I might think twice before giving them to a tween because of that third book.
I thought that all of them were rather dark. My kids' biggest issue with the third book, and I tend to agree, is that it didn't end in a satisfying way (say the way the HP series did) and the author tended to change/develop the characters' core personalities in a way that didn't seem totally believable (Katniss especially). I can certainly see the types of things that go on in those books modifying one's world view, but it seemed like the main character become someone she wasn't likely to be.
Sorry for the derail and I'm sure that not everyone will agree with me.
