I honestly do not believe Kalista is the Ruined Queen. Typically, though in fiction this is rarely the case, royalty are not on the front-lines leading a battle, unless they are gods/demigods, in which all bets are off. Kalista looks armed and ready for a fight, her armor is simple, something good for mass production, rather than ceremonial. Compare and contrast the armor of
against someone like Helmet Bro. If anything, she was part of the Shadow Isles standing military, perhaps even in an important position within its hierarchy.
Heck, she might have been among the King's personal guard, alongside Hecarim, who carried out his King's will to the letter, despite this leading to the Shadow Isles' doom. Hence, Kalista's accusation of betrayal; Hecarim's blind loyalty ironically facilitated the fall of the Ruined King.
For me, I would not be surprised if the Ruined King looked for a way to open the divide between life and death for the sake of a loved one, whether it be for a wife, son, daughter, or sibling. And Mordekaiser, being the first undead, helped him discover the means to do so, but took advantage of the King's trust, twisting whatever ritual was performed to spread death, rather than opening the way to it. He manipulated Hecarim into believing all was according to his King's will, his deception only being apparent once the Black Mist covered the isles with death and decay, and by then, it was too late.