I need to know only 1 thing, who killed Mauvole - Ashe's and Sejuani's sister?

Nobody, unless we're going to assume there was a cover-up.

The elders of her tribe claimed that she died of natural causes.

Wasnt Lissandra the name? Or is Lissandra Mauvole's sucesor?

It's safe to assume that Mauvole simply died of old age, which is a pretty fantastic feat when you consider that she was one of three leaders of a civil war.

I feel some unbalanced things here. Just look: Demacia vs Noxus, Zaun vs Piltover, Bandle City vs Bilgewater and Freljord vs.....
It's not honest if 1 small tride fights vs 2 trides + barbarians+ bears + yeti
If Mauvole died cause of her old age, shouldn't she have a daughter, wich could be her true successor. Who is this fuc..ng Lisandra, who betrayed the traditions of her tribe....
So much questions and no answers. Sorry guys, i just waited 3d Freylord queen very much and was shocked little bit when heard about her death((((((((((((((((((((((((((