Confused on ending

"i just finished watching this and i'm really confused"

If it made sense, it wouldn't be anime.

You can't understand this movie's ending without also understanding parts of the beginning and middle.

Despite what some commenters have said about anime being inherently incomprehensible, this movie consists of fairly simple, though hopefully not predictable, twists and turns. I may have missed some details, though think I figured it out correctly. Here's my (spoiler-filled) synopsis:

Kasumi (the twin sister with a suicide attempt scar on her wrist) refuses at the last moment to accept that she will be preserved in cold sleep, leaving her twin sister Shizuki to almost certainly die of the medusa virus, when Shizuki’s A pinky promise overture is slapped away and degenerates into a cliff’s-edge struggle, in which Kasumi falls to her death on the rocky Scottish beach below. In her anguish over Kasumi’s death, Shizuki exhibits the special "wishing" psychic ability previously shown by the (possibly) deceased Russian girl Alice, and is taken into the innermost (or rather, uppermost) lab of the Venus Gate castle and subjected to experiments. During these experiment, about 48 hours after the 160 patients are put into their cold sleep capsules, the power outage caused by Peter Stevens's sabotage occurs then, allowing Shizuki's “manifestations” to run out-of-control, filling the castle with monstrous plants and animals, and releasing the sleeping patients.

Via her obiquitous thorn vines, Shizuki retrieves Kasumi's corpse, placing it at on the dais in the center of "area 0", then makes a duplicate of Kasumi, placing it in one of the cold sleep capsules. Marco and Stevens both witness the initial intrusion of the vines into the cold sleep chamber, but or knocked senseless before seeing duplicate Kasumi places in the capsule.

This duplicate Kasumi is the main narator of the movie. Her memories differ from the original only in the moment of the cliff’s-edge pinky-promise, where she remembers accepting it, while in reality, she rejected it, leading to the struggle and her death.

At the movie's climax, duplicate Kasumi reaches the dais at the center of area 0, expecting to find and wake a sleeping Shizuki. Instead, she discovers her original's corpse, distinguished by the suicide attempt scar on its wrist. Ignoring warnings from Shizuki, now a part of the whole monstrous plant-being, duplicate Kasumi gains the true memories of her origin, and is then ejected from area 0, and with Marco’s aid, the whole collapsing thorn vine/dragon/ castle.

Afterwards, In their final conversation, Marco consoles duplicate Kasumi with speculation that all people are dreams of an "inconceivable, ancient being", requesting that she go on with her life, untroubled by her knowledge of her origin. She seems to take his advice to heart, and is seen in the movie's final scene walking away from the ruined castle had in hand with young child Tim.


One plot twist continues to mystify me, even after rewinding and watching its scene several times:
Why, or does, Marco die in the penultimate scene?

He doesn't appear to have been petrified, or mortally wounded, but rather to just stop moving, perhaps satisfied that he has finished his mission.

Help deciphering this scene, or agreement that it simply isn't adequately scripted or drawn to make sense, would be appreciated.