The major negative role that these two sisters play is their role in destroying King Lear, their father. They manage to persuade him to give them the kingdom and to cut off their sister, Cordelia.
I think that they act this way because they are selfish and hungry for power. They do not really care about their father. All they want is to get more power and wealth for themselves. Because of this, they tell him what he wants to hear instead of telling him the truth. But once he gives them what they want, they are vicious and horrible to him.
The most obvious answer to this question has already been put forth by pohnpei397: Goneril and Regan serve as the means of the undoing of King Lear. Yes, they wanted to play this negative role in pursuit of money and power.
It is no wonder that Goneril and Regan are so inclined. In fact, Goneril and Regan, rather then being the "wayward" daughters of their father, appear to be the ones who are the most believable in this story. It is Cordelia whose behavior seems strange. In the face of what we know of the family's history, Cordelia stands out as quite unusual, indeed.
In the opening act of Shakespeare's play, it is obvious that the king is characterized as the embodiment of the kind of negativity that shows itself in Goneril and Regan. The siblings have obviously been spoiled -- given everything that they might have desired. Like their father, the desires of these two daughters are superficial and shallow in terms of what the true treasures of life really are. Like their father, playing at his utterly immature and hapless "contest," the two daughters emulate his negative behavior by concocting "contests" of their own, sharing the king's same impoverished spirit. Their behavior is completely consistent with their role model.
The problem, therefore, is Cordelia. Is there anything in the play to intimate that Cordelia has been given any other moral and spiritual instruction than that of her father and, hence, her older sisters? If there is no evidence of another influence in the life of Cordelia, how did she become so honest -- so transparent, and, as such, so courageous? How could she have developed such a moral and spiritual character that persuaded her that it was worth the sacrifice of her life to redeem her father? Is Cordelia a female "messiah figure"?
Look for anything within the play that would give credence to the "messiah" idea, for, instead of it being unusual that two daughters would play the negative roles in the destruction of their father, whose basic character was identical to their own, it is even more unusual that, amidst the environment of KING LEAR, a positive character such as Cordelia could exist.