Surely the least you should be able to expect is that someone will die?
If the blurb on the cover of a book says that people will die, then that’s what will happen. If ‘not everyone will be alive,’ I expect this to cover the good guys in the book. If it was only the case that a bad character snuffs it, then we are hard-hearted enough not to mind too much.
I mean, it’s obviously great if none of your beloved regular characters die in the course of the book, because you prefer them alive and kicking. And a little threat on the cover is not necessarily a bad thing; it makes you definitely* want to read the book, and you will be a little afraid, and then you will heave a sigh of relief when it turned out that they twisted the truth.
But should they lie?
You can write things in such an ambiguous way that the reader can’t be certain. They will think it’ll be all right, and they will hope. But they won’t know. When I write reviews I try and hint that you can’t be totally sure all will be well. But I work on staying truthful, and on there being no spoilers.
The thing is, if it’s a book not intended purely for adults, then most likely the characters you care about will live. There are unspoken rules.**
*I remember when the Retired Children’s Librarian told me she stopped watching NCIS halfway through season three. In fact, she switched off partway through an episode, because when the knives came out, she simply grew too frightened. To be helpful, I pointed out that they were unlikely to kill a main character just like that. The knives were a threat intended to worry you a little, and make you wonder how they were going to get out of this situation. Not if.
** I know. What about Lupin and Dumbledore, Fred Weasley, Dobby, or even Snape?