Honest answer, if you want full spoilers: the character who leads the assassination in the books hasn't been introduced except as a background extra (it's several minor characters who all ultimately stab him together though). Everyone figured they've been building up either the kid or Alisser Thorne to do it instead, but they've been laying it on really thick with the Olly foreshadowing.
The reason is because Jon plans the march the Wildling army on Winterfell to kill Ramsay and rescue his sister from the Boltons. He receives a really awful letter from Ramsay basically gloating about all of his crimes and threatening the Watch that goads him into doing this. Everyone views it as a betrayal of his vows, and knows that if he fails the Boltons will destroy the entire Watch, so they stop him. This is kind of weird for the show, because it's Jon's allies who do it, and the show doesn't really have a character who fits that mold.
If Alisser does it, it'll be really hard to separate it from the fact that Alisser has hated him all along, and if Olly does it, it'll be really hard to separate from the fact that Olly really, really hates Wildlings. Olly doesn't exist in the books, btw. Of course, they may be planning on changing the reasoning for the show.
Oh, also everyone who's read the books is pretty sure that Jon gets resurrected through some sort of magical shenanigans involving Melisandre, Bran, warging into his wolf, or some combination thereof, so don't worry too much.