AJ is trying to kill them all, she is the tratior
I think it's fluttershy
Snowy may have a point. Applejack's the one mind we've read the less, and it's been stated that she's extremely greedy.
The death of Celestia might bring an economic depression unlike anything ever seen. Dead Celestia = Economic Depression = Less Money for Applejack.
Also, a good reward from the princess for helping her bag the Betrayers could seal her affiliation.
Confusing panel order here.