I think that Katherine is just the next logical, if there ever was a logical death, in the series. She got Jeremy killed for a cure that she is not even in possession of anymore. Klaus refused to give her the freedom and Silas ended up with the cure anyway. I don't think it's out of the question to say that Katherine is likely to die, but I don't know that Elena could do it on her own. I believe there is no way, not in a million years, that Bonnie would make Katherine truly immortal. She is going to renege on her end of the bargain. That could play into it, should Katherine actually die.
It was a good theory, but I guess not, Bonnie will not interfere. Katherine will die by the hands of Elena, they will fight and accuse mutually about everything that has happened. I think Elena is going to take advantage of the vulnerability of Katherine and take advantage of it. It is a pity this character dies, I really like her, by the your history, even by their evil deeds, by the their loves, everything. I am preparing psychologically for her death, I don't want to stay as I was with the death of Kol. She had a lot more things to be explored, under that cover of coldness, she demonstrates. But as they say hope is the last to die, let's wait for the end.
I think too that Kat will die in season finale. But also I think it's a very stupid idea, because Kat much older than Elena and stronger. Sorry, but both of the Salvatore brothers can't beat her and she was run from Klaus over five hundred years and now some vampire baby will kill her?