In the first instance, Estella has been shaped into the harsh insensitive girl that Miss Haversham has made her to be. Miss Haversham takes this girl into her home (by way of Mr. Jaggers' arrangements, as revealed near the end of the novel). Miss Haversham is a very bitter old woman, jilted in her youth, who sits in the rotting filth of her many years of isolation. She has lost all hope of believing in the power of love again, so she torments the girl into becoming a tool to break hearts on her behalf.
When Pip comes onto the scene, Estella is promenaded in front of the boy to aggravate him into tears. In flitting moments, one can see a few glimmers of a kind heart in Estella, confirmed in the latter part of the novel; however, she is churlish, condescending, overbearing, humiliating, insulting, and otherwise a hideous girl due almost entirely to Miss Haversham's will to crush men, even such a one as young Pip.
Finally, she is well educated, very beautiful, and ultimately dutiful to her destiny.