For five seasons, Samwell Tarly sticks by Jon's side, trying to keep up with him as best he can and always coming to his defense. It's sad to see them say goodbye in the fifth season finale when Sam sets out for Oldtown with Gilly, but even sadder when we realize that he wasn't there to defend Jon from being killed by his own men.
While Alliser Thorne's betrayal of Jon is no surprise, Olly's is a bit more unexpected. Sure, he was upset with Jon for sympathizing with the Wildlings, but helping to murder him is extreme. It's likely his decision was influenced by his conversation with Sam earlier in the season, when Sam explains Jon's decision to help the Wildlings and says that sometimes unpopular decisions have to be made.
However, Sam's next words jump out as foreshadowing about his friend. He tells Olly not to worry about Jon, because "he always comes back." Jon is extremely lucky when it comes to surviving things, but in season six he comes back one more astonishing time — from the dead.
