It's been a while since I really enjoyed a YA book. Somewhere along the lines I just got incredibly burnt out on them and after a while they all seemed to be the same book, just with a few differences. Same insta-love, same "but I'm QUIRKY" type of characters, and so on. If you've read YA fiction, you'll know what I mean by all of that. This book is fairly surprising in that while Vega does go for some of the typical tropes, she uses them in a pretty effective manner. 
 What makes this book work and seem a little more spooky is how easily much of this work could actually happen at any given point in time. Sure, sure, this book is obviously a work of fiction, but a look at the news shows us how a group of girls can try to commit a murder to please Slender Man and how various different people have caused murder sprees because they believed that it was God's will. There are other elements to this work, but the creepiest parts of Merciless tend to center upon how awful group mentality can sometimes be. It all depends on who is sitting in the driver's seat. 
 There's a bit of a twist at the end and die-hard readers of horror will likely see it coming, but knowing what is likely to happen doesn't really take away from the fun of reading the scenarios as they happen. If anything, picking up on it earlier on in the book helps make the little hints and foreshadowing all that much more creepy. 
 I do have to warn parents: if you aren't comfortable with acts of violence, some of which was done by a dangerously zealous Christian, then you might want to skip this read. Sex is mentioned, but the main thrust here is violence. It's a good stepping off point for discussing how religion can be the impetus for incredibly wonderful acts, but also incredibly awful ones. (Granted there is the twist, but it would make for an excellent way to spark a conversation.) 
 Overall, this is pretty effective and I can see where this would make for an excellent horror movie. I'd watch it, in any case.