Why was Jon Snow killed?

During the elections Jon snow won by only one vote. Ser Alliser had many supporters who didn’t want Jon Snow to be in charge.

The Hatred Ser Alliser Thorne had for Jon Snow might have motivated him to plot to kill him.

Jon Snow ordered a mission to rescue the remaining Wildlings. This mission made many of his men question his loyalty.

During the mission to rescue the Wildlings many of the Night’s watch men died. This fact made Jon Snow lose some of his popularity.

Many of the men weren’t happy with the decision to allow the Wildlings to pass the wall. (See Game of Thrones Season 3 Summary)

Many of the men questioned Jon’s loyalty to the crows especially that he has a certain level of affection to the wildlings.

Olly ,who saw his parents getting killed by the Wildlings, couldn’t accept the fact that Jon Snow allowed them to pass through the wall. (See Why Game of Thrones Season 5 is bad?)

The death of Maester Aemon and the fact that Samwell left resulted in leaving Jon snow with almost no friends at Castle black.