Ok this might be a couple of months too late but I needed to simmer the facts and ides properly else I might make some contradictions. Anyways, one of the few champions that are currently shrouded in mystery is Kalista for except for the Q and A and her voiceover we know nothing else, anyhow I'll just start and list down things that we know. Kalista - was once a legendary warrior in life and died trying to prevent some disaster from happening. The spears lodged on her back serve as a reminder of how she was murdered and betrayed. Now spending the rest of eternity as an undying revenant out for revenge.
Hecarim - One thing that happened to Hecarim before Kalista's reveal was that his page became shrouded with the Black Mist which means that he is one of her viable targets because he betrayed someone, but who? The Ruined King - was once a noble king but was ruined somehow. Ruled the Shadow Isles before it was corrupted and it seems he caused the corruption by ordering his sorcerers to "break" the barrier separating life and death. Was overcame by grief. His sword (heh) was once a symbol of hope but now of despair.
Evelynn - A mysterious assassin with a mysterious heritage.
Maokai - was once a peaceful nature spirit but was trapped inside a oak tree while trying to preserve the energies of the forest. Was corrupted in the process.
Ok now that I'm done with that here are my speculations.
I'll start from the beginning of the timeline of my theory, we know that the Ruined King ruled the Shadow Isles before it fell but was overcame by grief, death of a loved one might be the cause, of a wife to be specific or a daughter to be more likely. I believe that The Ruined King lost his daughter somehow and was overcame by grief and clouded his judgement leadinng him to order the breaking of the veil which lead to the corruption of the Shadow Isles. But how did he know that it was possible? Maybe someone tricked him, someone took advantage of the king's demise, someone who needed to break that barrier or more likely wanted to cross the veil himself.
Moving on to Kalista, one thing that I noticed when Kalista was revealed her spears looked familiar and it was because it reminded me of Hecarim's spears exactly down to almost every curve of the blade and when I studied it further more it seems that Kalista's helm resembled Hecarim's but that it is you removed the hair from Hecarims. I believe that Hecarim betrayed Kalista, he ordered his cavalry to kill Kalista but for what reason? I believe that Hecarim was loyal to the king and was allies with Kalista, or even best friends and to even add more salt to the wound, they might have even been lovers, Riot never did state that Kalista was single but when they were asked they answer with only one word - Ha!. Anyhow, Hecarim and Kalista most likely had a deep connection but when Kalista tried to stop the king from doing the ritual Hecarim saw it as an act of betrayal to the king and he had to kill her.
But how did Kalista betray the King? Because she knew somehow that whatever the king was dangerous, maybe someone like Maokai the friendly nature spirit told her that it was endangering the forest or maybe someone was someone acting like a disney villain and sang a song about death (ha!) and Kalista overheard him somehow.
Now all of the characters have been connected except for Evelynn so why did I add her? Because she is the loved one that the Ruined King lost. Whatever her connection to the king was, he loved her so much that it made him conduct the ritual just to see hear live again, even at the cost of this entire kingdom. But why evelynn? Because in her new lore tidbit it claims that she had a mysterious heritage and Riot did this one on Sivir and it turned out she had a royal heritage so I have this nagging feeling that Evelynn has one too.
With the Ruined King left in despair someone saw this as an opportunity, someone who needed the king's aid, someone like Karthus. I believe that Karthus' overwhelming desire to meet death itself lead him to study about it and it proved worthy but he lacked magic, power and resources but with the king's demise he saw knew where to get it. He gave the king a deal for he knew a way to bring her back somehow and this lead to the fall, or birth, of the Shadow Isles. Kalista on the other hand knew od the tradgedy yet to come and summon her men to stop the king but Hecarim ever loyal to the Ruined King stopped her. Kalista was overpowered and was left alone amidst the bodies of her allies and was given a chance to turn back but it seems that only death would stop her from her goal, Hecarim had no choice but to deliver the final blow. An entire army's worth of spears pierced Kalista but it was not her end for the King's ritual was not undone and turned everyone undead including her.
tl;dr Hecarim betrayed Kalista.