[It's for Barty to discover whether such ambition is admirable, or even wise. Bellatrix survives the Dark Lord's tutelage — if it isn't too generous to call it that — by quieting her enthusiasm as best she can. A devoted servant should be sparingly curious. If their Lord ceases to teach, she wisely ceases to learn. Her Occlumency is the perfect example. Regardless of how she means to use it, it's impossible to refine one mental skill without gleaning something of the opposing art, and yet, she's almost wholly undeveloped as a Legilimens. She has, instead, a keen sense of intuition, undermined somewhat by her haste to judge. The Dark Lord shapes her ignorance as well as her knowledge. She's aware of this. Before improving herself, she questions whom it will benefit. She doesn't do so selfishly. Her sadism, on the other hand, is sincere. Inflicting pain is her dearest and most personal indulgence.

As for the nature of Barty's cruelty: she might know before he does.]

That you'll have to tell him yourself. [She shoots him a chiding look, but there's no strong reprimand behind it. He's seen the manic intensity in her eyes when she has a real grudge, and this is nothing. Still, which Lestrange was his remark designed to offend? Not her, by the most direct interpretation, but Barty risks being impudent in order to hit the other target from afar.]

You make it sound as though I insist on being pleased. I can please myself.

[It will have nothing to do with her, if the boys have a spat. She reasons that it would be a simple matter of clashing egos. They might end up dueling just because Barty's obviously not afraid to duel, and Rodolphus would make some stupid effort to put the fear—or respect, or whatever—into him.]

I suppose you think you're the stronger wizard. [She phrases it as an observation, not a question.] So bold.