Beloved is the representation of guilt and sadness Sethe carries with her permanently about what she has done to her children to keep them from slavery. Beloved's constant need for sugar is the representation of Sethe trying to make up for what she's done, to placate and spoil the Beloved she has denied life to, as it were. She spends money foolishly, money she doesn't have, and acts out of guilt and shame at Beloved's every whim and need because it is the only way her character can live with herself and the crime she's committed.
While, as readers, we sympathize with and feel the cruel victimization Sethe has gone through in her life, knowing the ugly truth at the end of the story makes it difficult to forgive her - as difficult as it is for Sethe to forgive herself.
Beloved is her permanent, immature and enraged reminder.
I think that you will find different points of analysis to this question. I have always felt that the character itself is a challenge because there are so many emotional connections and layers to her. One cannot overstate the importance that if we presume that Beloved is a spirit, or at least functions like one to Sethe, that she carries with her the two harshest of psychological conditions. She is a child of slavery and is a child of infanticide, of both social and personal rejection. Beloved is a character whom the narrator describes as someone for whom, "the sweet things in life was what she was born for." Contrast that with the social reality of slavery that would have robbed her of any joy and a condition that compelled her mother to kill her, confirming the theft of joy. Perhaps, the need for sugar is a compensatory mechanism to help bridge the gulf between the sweetness "she was born for" and the bitterness in which she lived. Along those lines, Beloved flies into her rage for not having sugar because this "sweet" was denied to her in life. She feels that it's Sethe's obligation to give it to her in large quantities now that she has returned. It's no different that any child wanting something that was taken from them. The destruction that Beloved brings to bear when denied sugar could also be her voice, silenced for so long, finally speaking out about the injustice that she endured. In this vein, Beloved becomes a metaphor for children of slavery as well as those who have been robbed of what was rightfully theirs. This is why Beloved becomes such a challenging character to analyze, for the sins of politics and the sins of the heart, of justice and of love, have converged in her essence, her state of being.