It is clear that the dramatic change in Regan and Goneril that is displayed in the play as they move from appearing to be loving and devoted daughters in Act I scene 1 and then become selfish, power-hungry individuals after this indicates the true nature of Regan and Goneril, who are used in the play to personify evil in its varying states. Consider the way in which they turn out their father onto the wilderness in the storm at the end of Act II and then the glee with which they put out Gloucester's eyes in Act III.
Clearly, as far as their motive is concerned, it is their desire to gain power and influence, whatever the cost, that dictates their behaviour. In Act I scene 1 therefore, it is clear that they have to play the role of devoted daughters in order to gain the power and land that they desire. When they have played this role, and have the power that they have cherished, there is no need for them to continue that role so they dispense with it quickly, disposing of their father without a second thought to suit their own selfish natures. Their motives then are based on nothing more than power and selfishness.
Of course there is the motive for gaining power, however the lack of a motherly role within their lives is also intresting. One could argue that the pair, who have had to live with the tyrannical figure of Lear thats presented in Act 1 Scene 1, are justified in their treatment of him as they have had to endure the masculine, egocentric rantings of Lear for their lives, they have first hand experience of how his emotion can get the better of him and throw him into violent tantrums - maybe their cruel treatment is justified, as their past experiences have proved that Lear needs a strong hand to deal with.