Who was responsible for Elder Souma's death?

Confessions of a Broken Blade: Part I - Universe of League of Legends
In the wake of war between Noxus and Ionia, Riven is accused of a heinous crime.
Based on the revelations in the linked story, who do you think was actually responsible? If you haven't read it, please do. Remarkable narration by RIOT. {{champion:157}} {{champion:92}} {{sticker:sg-lux-2}}
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