I do have a bit of a problem here. If it goes as far as Anna killing Elsa it seems like her love for her sister didn't overcome evil's influence. On that note how would Elsa come back? Do you have any more specific plot points here? Any more specific ideas?

I'm sorry for my late reply

the main theme for this kind of conflict is about Forgiveness, really. as fro Anna not overcoming the influence, this is more for plot's sake because the lesson imparted here is that every relationship we have has its negative moments, but we must not outweigh the bad times against the good and let it destroy our relationship; that every relationship needs forgiveness, healing, and starting over. Frankly, I see Anna as a walking emotional time bomb, just like her sister; She just is able to control them more effectively, and Anna is frankly better on 'putting on a mask' than Elsa was; concealing her hurt and insecurity behind a mask of cheerfulness, optimism, and bubbliness. Anna may not blame Elsa for her isolation and feeling of rejection and loneliness on a typical situation (post-frozen) but who's to say that Anna may harbor deep resentments and hate against her sister for ignoring her for 13 years that were subsequently pushed down the back of her mind after Frozen's events? We've seen some parts of these feelings from Anna from her outburst at the coronation scene. That these feelings go un-addressed after the Great Thaw and even during Fever means that without direct confrontation over these feelings, they would bound to spill out in some way or another, especially in stressfull situations. You can't realistically have the sisters get along all the time; conflicts will arise that puts their relationship to the test. Anna may not ever openly display to her sister how much hurt she felt, but a catalyst like a spell could have disastrous results. and interesting fodder for cinematic drama may i add.