Well-placed qualms with the fourth episode—titled "The Last of the Starks"—aside (though, we agree they may be hard to overlook; the episode is currently the worst-rated GoT installment of the series on IMdB), Missandei's undoing foreshadowed another character's devastating fate: her Dragon Queen, Daenerys.
This "dracarys" statement seems to push Dany over the edge. We've already seen her second dragon—whom she thinks of as her child—swept aside by Euron earlier in the episode. Now, it's as if the dragon queen is foaming at the mouth. Missandei's one-word statement seems to spur Daneerys on: "Let them burn," she seems to tell her queen. "Avenge my death. Burn them all." Sound familiar?
It should. Those words are what Daenerys's own father, the "Mad King" Aerys II Targaryen, said during the sack of King's Landing when he ordered his hand Ser Jaime Lannister to light the wildfire that was hidden underneath the city in what would've caused hundreds of thousands of innocent civilian deaths. As the "Mad King" says this, he succumbs to his own madness, and continuously repeats, "burn them all..burn them all...burn them all..." until he's killed by Jaime.
And here we are, back on the parapet of the gates of King's Landing. Missandei is on the verge of death (we can see it in her eyes she knows this to be true; Grey Worm knows; Dany knows; everyone knows she's a goner). In her last plea, she tells her queen, "Dracarys." Daenerys's eyes are like two flaming orbs as the Mountain's sword swings through Missandei's neck.