If she is going to do that after you break off from her, meaning, if that is within her character to backstab, it is possible that she has already done so while you were with her.

I once had a hard time breaking off from someone in fear of retaliation. But even when we were close, she retaliated anyway whenever she didn't get her way. Then I thought:

Whether I remain close to her or not, I can't avoid retaliation from her.
Then it's better that I at least do not have her in my daily life.

But it is still wise to minimize retaliation as much as possible by not engaging with her.