to be married to him.”

Prior to this discussion, Pip and Estella had been talking about her nature and capacity to love. This character Estella may indeed be marrying Drummle as a service to Pip. She does not love Pip like he wants to be loved by her, but she may have the capacity to understand that she could not fake it just to please Pip. Notice, in this discussion about Drummle, she never says she loves Drummle. In fact, Drummle fits for Estella just because he is of high social standing. Pip aspires to be of high social standing, but it is a stretch, and after learning of his benefactor, it still may be further off.

Another direct perspective or interpretation could be that Estella marries Drummle because throughout the novel she has been learning to seek revenge on men for Miss Havisham. Pip has been her life's work. To marry someone else would be the ultimate injury to Pip. Pip points this work out between Havisham and Estella in these words:

Estella, dearest, dearest, Estella, do not let Miss Havisham lead you into this fatal step. Put me aside for ever—you have done so, I well know—but bestow yourself on some worthier person than Drummle. Miss Havisham gives you to him, as the greatest slight and injury that could be done to the many far better men who admire you, and to the few who truly love you. Among those few there may be one who loves you even as dearly, though he has not loved you as long, as I. Take him, and I can bear it better for your sake!

Great Expectations is a great novel to learn life lessons from. Whether the intention of this marriage is to point out that marriage should not be done for convenience or to illustrate Pip's problem of putting all of his effort into one woman, this much is clear: Estella does not love any man, but for social standing will marry a man who did not grow up in a forge.

Estella has been brought up by Miss Havisham as a lady, to believe that she must take her place in the world as a member of the social elite. As such, it's necessary for her to bag a rich, eligible bachelor, someone who can keep her in a style to which she believes herself entitled. Bentley Drummle appears to tick all the right boxes. He may be a thoroughly nasty piece of work, a loudmouth braggart and bully, but none of that really matters to Estella. Miss Havisham has taught her that men are there to be manipulated and controlled to get what you want in life. Love doesn't enter into marriage at all, and so Estella's under no illusions as to what getting hitched to Drummle will entail. However, Estella's superficial understanding of human relationships receives a rude shock when she becomes Mrs. Drummle and finds out just how abusive her new husband really is. It's only when she emerges from the shadow of Miss Havisham's baneful influence that Estella's finally able to comprehend the true nature of love.

It is ironically interesting that Bentley Drummle becomes the parallel of Miss Havisham's suitor, Arthur. Named the "Spider" by Mr. Jaggers, who probably has sensed this resemblance, he is described by Pip as "idle, proud, niggardly, reserved, and suspicious," The boorish Drummle cares for no one but himself, and, in this respect, he is somewhat like Estella who "has no heart."  For, when Pip visits Miss Havisham in Chapter XLIV in order to supplicate a favor for Herbert, he finds Estella there and declares his love for her. Estella looks at him strangely,

It seems,” said Estella, very calmly, “that there are sentiments, fancies—I don't know how to call them—which I am not able to comprehend. When you say you love me, I know what you mean, as a form of words; but nothing more. You address nothing in my breast, you touch nothing there. I don't care for what you say at all. I have tried to warn you of this; now, have I not?”

Then, when Pip asks if it is true that she is to be married to Bentley Drummle,

It is quite true,” she replied, referring to him with the indifference of utter contempt.

“That you encourage him, and ride out with him, and that he dines with you this very day?”

She seemed a little surprised that I should know it, but again replied, “Quite true.”

Pip protests that she should not waste herself on such a lout, but Estella counters,

"On whom should I fling myself away?” she retorted, with a smile. “Should I fling myself away upon the man who would the soonest feel (if people do feel such things) that I took nothing to him? There! It is done. I shall do well enough, and so will my husband. As to leading me into what you call this fatal step, Miss Havisham would have had me wait, and not marry yet; but I am tired of the life I have led, which has very few charms for me, and I am willing enough to change it. Say no more. We shall never understand each other.”

Deprived of developing the natural feelings of love and kindness, Estella has wearied of being cold to Miss Havisham and the others near her.  So, she turns to a brute such as Drummle, who perhaps may provide a new type of challenge for her cold heart.  In fact, she does defeat him, as well, although he is the Spider who catches part of her fortune:  he dies.