Bellatrix also wants to leave, but not as much as she wants to drink, so she will initially go back and forth between refilling her glass and clutching Narcissa's arm in a way that is 100% sisterly rather than using her as a human buffer. Eventually, once she's had enough wine, she will at least say hello to people she doesn't hate if she bumps into them but for the most part she is people watching from the edge of things with that whole hyper-intense gaze thing going.