According to oxford dictionary, to backstab means
The action or practice of criticizing someone in a treacherous manner while feigning friendship.
while to betray means
Be disloyal to
however, I have seen backstab and betray being used to mean "doing something that harms your friend"
I do not understand the difference. For example, A trusts B with his bank account, but B steals from A by spending the money in A's account. Is that considered backstabbing as well? I understand that in this context, B has betrayed A's trust.