she could easily change her mind on killing him. all this time she has just wanted vengeance and she believes he's a monster. then when finally confronted with him, and told the most perfect thing, she realizes that what she's doing is wrong and that it's keeping her from doing what is more important.

there is a lot left for stannis. he's pretty much the only viable character to lead the nights watch, and him doing so wraps up his entire arc quite nicely. whereas if he's just dead now, most of the time spent with him was a complete waste and should have been cut from the show.