Zombie Stage is a role reversal
(Yes I'll shut up after this)
The moment they lose the other to a place they can't follow
Sua loses Mizi to the kpop industry, a place she can't follow because she couldn't pass the audition. Even more than that, the kpop industry is the place that tore Mizi down so completely she became a soulless husk of herself, which is the source of their emotional rift and the start of Mizi pushing her away.
Also, in a way, Mizi died metaphorically after her debut, as shown by her dead eyes and empty smile in all of her scenes after.
Mizi loses Sua to death in Round 1, a place she can't follow in any meaningful sense.
The moment they see the face of the person they love hurting and in need of saving
Mizi is the one who needs saving.
Mizi's suffering is the main focus in the scenes depicting their childhood, while Sua is just happy to be with Mizi (as clearly seen in the frame where Sua leans in to kiss a bruised Mizi for the first time).
Mizi's active suffering in the kpop industry is also one of the main focuses of the video.
Sua is the one who needs saving.
Many of Sua's childhood art depicts her dead-eyed and presumably dissassociating to escape from a painful reality. In contrast Mizi was usually depicted just being happy to be with Sua.
Sua's loneliness and tendency to bear her suffering alone without sharing it with others is a major character theme.
The moment they try to reach the other and help, but are pushed away (literally)
Sua desperately wants to help Mizi, but Mizi pushes her away because Sua will never be able to understand what she went through in the kpop industry and that fact makes her unbearably hurt and angry and lonely.
Mizi desperately wants to reach Sua, but Sua pushes her away because she needs to go through with her plan for Round 1. Sua is also under the assumption that Mizi doesn't know/wouldn't understand because Mizi's pretended not to know this entire time, which in turn has subjected Sua to the loneliness of enduring all of this herself.
The moment they smile blissfully at the one they love, despite their partner's pain
Sua smiles in this scene because she's finally found a way to stay with Mizi. The zombie virus and Zombie Stage has become a "paradise" for her because despite the horror, it allows her to reach and be with the one she loves.
So when she reaches out to kiss Mizi who's still trembling from the pain of decomposing as a zombie, it's with a smile of pure bliss.
Mizi smiles uncertainly but eagerly as she reaches out to Sua while they bathe in the same tub, despite Sua showing extremely concerning tendencies, bringing Mizi's hands around her neck as an unspoken display of the pain that has driven her to arrange her own end in Round 1 at Mizi's metaphorical hands.
The moment they remembered their happiest memories during the climax of the story
For Sua, that moment was practicing together with Mizi in the studio, before she debuted.
For Mizi, that moment was the crystallization of their greatest wish, their final duet.
The moment they danced with their lover's (un)dead body
Tl;dr Zombie Stage is a role reversal bc Mizi is the one suffering and in need of saving while Sua is the one chasing after her shadow after losing her.
