ABSTRACT: The writer explores the possible relationship between Bellatrix and Voldemort.
Let me begin by saying I knew Snape's secret before Deathly Hallows came out. I have read and re-read the Harry Potter series over and over many times. If there is one thing that I know about Jo's writing is that nothing is an accident. If Jo has made a point of saying something at least twice, then there is a reason. What gave away Snape's love of Lily was actually something that Jo never explained in DH. But it was Slughorn mentioning in HBP that Lily was a Potion's whiz. That started the wheels turning and made me re-read everything with the idea that Snape's relationship with Lily was the key. With Jo's tendencies in mind, I read into something interesting about Bellatrix.
In reading Half-Blood Prince, there were two things that stood out to me. The minor warning bell to me was in Spinner's End and the conversation between Bellatrix and Narcissa. When discussing Draco's mission Bellatrix went on to say that had she had sons, she would have gladly handed them over to Voldemort's service. It was her choice of the plural that got my attention. It would seem that a woman with no children would say that sentence differently. "If I had a son" would seem more appropriate for a woman who doesn't have a son and isn't expecting one. But that sentence isn't necessarily a give away.
The major difference and one that is different in the movie is Bellatrix's involvement in the first battle at Hogwart's. Bellatrix, Voldemort's "best lieutenant" (DH 737) was not there to ensure that her nephew was successful in killing Dumbledore. Why wasn't Bellatrix there? There's no explanation. Lucius is still in jail so he wouldn't be there...one can assume Voldemort released Lucius after Dumbledore's death.
Another red flag is how Voldemort handles Bellatrix's death. Notice the difference with how he treats Snape's death ("I regret it." DH 656) to Bellatrix's death where Voldemort screamed in rage and "exploded with the force of a bomb." (DH 737)
Then there are things Jo has said outside of the novels. She explained that Bellatrix's marriage with Rodolphus wasn't a love match and she married a true blood as it was expected of her (web chat interview). Rodolphus's disappearance in the second Battle of Hogwarts and in all of DH isn't explained either.
And there's the thing that Jo DID say in the novel more than once that is the big warning flag. Jo states in DH that Bellatrix spoke to Voldemort as if he were a "lover." Voldemort tells Bellatrix at Malfoy Manor "No higher pleasure...that means a great deal, Bellatrix, from you." (DH 9) and after Voldemort regains conciousness in the Forest after using the killing curse on Harry (DH 724).
So how do I add these things up? My theory is that Bellatrix had a child with Voldemort. I beleive that she was pregnant at the beginning of HBP and had him before DH. Notice how Voldemort warns Bellatrix that she must cut off the diseased branches of their family (DH 10). It seems very important to Voldemort that Tonks no longer exist. It would explain why Jo was hinting that Bellatrix was treating Voldemort as a lover, why Bellatrix wasn't at the first Battle of Hogwarts and where Rodolphus went (raising a baby that wasn't his, poor shlub). And if there is any way to continue the story of Harry Potter (which Jo has said is a possibility) it would be interesting to see if someone with soul-less parents or as close to soul-less as you can get is automatically evil.
Never used an Unforgivable Curse before, have you, boy? You need to mean them, Potter! You need to really want to cause pain — to enjoy it — righteous anger won't hurt me for long — I'll show you how it is done, shall I? I'll give you a lesson —
Bellatrix Lestrange Order of the Phoenix, Chapter 36, Page 810
J.K. Rowling once thought of writing the Death Eaters as the Knights of Walpurgis.