Ack, the pressure!
Ok, I was thinking about it tonight, and I realized I wasn't thinking like Voldemort. So I turned my mind in that direction, and I think I know where Bellatrix was.
She was with Narcissa.
Her purpose?
To kill her sister if Draco betrayed LV.
It's perfectly
evil ... Narcissa is the hostage (probably unknowingly) that Draco suggests in his conversation with Dumbledore in the Astronomy Tower. LV doesn't trust Draco to succeed -- in fact, probably expects him to fail (as suggested by Snape in Chapter 2). However, I'm sure LV has also considered the possibility that Dumbledore, if aware of the plan, might try to convince Draco to betray the Death Eaters. That would require a greater punishment -- the death of Draco's mother.
Now, why Bellatrix? Well, for two reasons:
1) I'm guessing the Dark Lord is aware of the events in Snape's house, and may not fully trust Bellatrix because she second-guessed him (not to mention her failure at the Ministry)
2) What better way for Bellatrix to prove that she is LV's most devoted servant, as she has claimed?
It's possible we won't find out the answer to this question, but if there is an answer, I hope this is it.