Bella is pretty much her old sadistic self in this film. She likes to think she's pretty important to the Dark Lord—and she's always very aggressive about trying to insert herself into or take on more importance in his evil plans (even when it's totally not welcomed):
BELLATRIX: My Lord, I'd like to volunteer myself for this task. I want to kill the boy.
Really, Bellatrix? If you'd actually been paying attention to what's been happening, you'd know that either Harry will kill Voldemort, or Voldemort will kill Harry. It has to be one or the other, according to that prophecy we learned about in Order of the Phoenix.
But that's Bella: She never misses a chance to butt in and (in her view) prove her faithfulness to the Dark Lord.
Bella manages to do some damage to poor Hermione toward the end of the film, when the kids end up imprisoned in Malfoy Manor. Bellatrix is totally enraged to realize the kids have the sword of Gryffindor, which is apparently supposed to be under lock and key in the vault at Gringotts, the wizarding bank.
So she tortures Hermione for a little while trying to find out how they got it. Of course, Hermione has no idea, since the mysterious silver doe brought Harry to the sword, and they don't know anything about how it got there…but Bella just keeps on torturing her anyway. Because that is just who Bella is.
Then she summons a goblin from Gringotts to torture him for more information, but luckily Harry gets him and everyone else out of Malfoy Manor before she can get too far with that plan.