Like lots of people in The Jew of Malta, Bellamira is just trying to turn a profit. With her body. She's the local prostitute, but don't look for a heart of gold here. While everybody else is freaking out about the Turkish siege, Bellamira's only upset because it's slowing down business, whining that her "gain grows cold."
Bellamira ends up doing a solid for Malta, even though all she wants is some of Barabas's gold. How? She pretends to be super into Ithamore so that he'll blackmail Barabas and share the proceeds with her and her pimp, Pilia-Borza. But she and Pilia-Borza eventually take the dirty info on Barabas to Ferneze.
Unfortunately, Bellamira turns out not to be so good at plotting. Not only does Ferneze her in prison, but Barabas manages to kill her with poisoned flowers. (Obviously.)
Here's some food for thought: like Barabas, Bellamira's main goal is raking in dough. Like Barabas, she comes up with an elaborate plot to get her way. So, what are the differences between them? How might a Jew and a prostitute be similar, in the world of the play?