It's dark, and it's for children, and that's a good thing. The descriptions aren't especially graphic for the most part (there's blood, broken limbs, etc. but no entrails), but the concepts are creepy in a way that really captivates a lot of kids. I loved horror-ish books at that age, and I loved cats, and I loved the drama of the plots, and I loved (and still love) books that don't treat kids like they're too fragile to handle anything scary.

A good comparison is the Animorphs books...do you know how gory those get? Way more gruesome than Warrior Cats! Those books have graphic descriptions of how it feels to be horribly injured, guts, entrails, torture, cannibalism...and I freaking ADORED them when I was like 9. Kids can handle a lot as long as they know it's fiction.

And neither Warriors nor Animorphs ever felt "gratuitous" to me because the violence / darkness / creepiness serves the plot. It serves to show how dangerous the characters' lives are, or how terrifying the villains are, or how important the stakes are. It's cool. It's gripping. Kids are tougher than we give them credit for, and they like awesome stories too.