So how do you see Arya doing this? Slipping a knife between her mother's ribs? Cutting her head off? Crushing her temple, shooting an arrow through her eye, hanging her, setting her on fire, some other sweet mother-daughter bonding moment I haven't thought of? Besides getting rid of a character you may not like, how is Arya's matricide better than Lady Stoneheart's revenge?
I'm sure GRRM will get rid of Lady Stoneheart in his own good time, but as much as he's given to horrible endings I can't see him having one of his favorite characters euthanizing her mother with an axe, or whatever, especially when there's so many other people Arya could be killing.