What is meant when Kirsten Dunst decrys her sister's plan in Melancholia as 'a piece of shit'?

What is meant when Kirsten Dunst decrys her sister's plan in Melancholia as 'a piece of shit'?

    Shit, that bodily scourge, is invoked by Dunst here in the plainest of terms. The image of a fetid turd is her riposte to the desperate resignation exhibited by Charlotte Gainsbourg, her sister. Faced with her own utterly inescapable demise, she devises a fallacious attempt to maintain face in front of her young son. However her absurd diorama stands deeply within the dark waters of Erasmus' definition of Folly, and it appears as if in her condemnation of this plan, Dunst has found an intuitive identity with Erasmus' conscious mind (despite finding themselves separated by the centuries, and that great ocean, the Atlantic.) “It’s a piece of shit.” Dunst cuts to the heart of the issue by this inspired proclamation. By her conjuration of scatological imagery in an eschatological context, she implies that their mutual destruction is a product of their own doing, just as excrement is the filth that we generate ourselves.
    To Dunst our moral filth is synonymous with our true feculence, and this correlates with her messianic portrayal throughout the film. Whether she is proposing they reduce themselves to coprophagy is up for debate, though it seems more likely that in her creation of the hallowed 'magic cave' she is creating her own Holiest of Holies within the natural world, and intends to keep it pure, away from the excrement she despises. Rather than embracing her effluence, she finds in it only revulsion, the very revulsion she applies to her fellow human beings. While Gainsbourg embraces the rose like scent of her shite, and in it's delusory miasma seems to experience a sense of expiation, Dunst rejects coprial methodology, and seeks rather to renege to a child like state in the presence of her nephew, a state where excrement remains a distant source of revulsion and wonder. This stands in stark opposition to her sister's fettishisation of her own filth, and at this moment the audience finally surrender all allegiance to Dunst; they wish only to follow her, and by transcending corporeal reality with her reject the coprophagous behaviour exhibited by all those who surround her.