Why is the Manga 'Death Note' rated OT?

I have read about 5 books of the death note series, and I am unable to find any extreme profanity, gory violence, , or anything that would really make it rated OT (for older teens). The only conclusion I have come to is that it's just a very creepy plot, and that it's a very mature concept. Any other ideas? Anyone who's read the series more than I have and knows what may have caused it to have this rating?