What exactly was Rose's plan at the end? She tells future Mary that she's going to wait for decades so she can kill her, which she clearly attempts, but she's also trying to kill young Mary at the same time? That doesn't make sense to me because if she succeeds in killing young Mary, then she can't kill her again in the future.
At least they followed the rule that a knife shown in the first act must go off in the third.
I think it was more so to threaten her. She never intended on killing young Mary; she merely used her as a tool to both torture and terrify adult Mary. At the end of the film where she, again, has kidnapped young Mary, is actually a misdirect. To make both the audience and adult Mary think hat Rose is going to kill young Mary. But it's just another form of mental torture.
My only qualm with the logic of the end is why the hell young Mary would so readily go back to the house of the woman who scolded her with boiling water a few days prior? You would think she'd be physically capable of defending herself or at the very least running away, especially given the conclusion.