It was believed by most, even by her own family, that Bellatrix was incapable of love. Sure, she called her sisters by the affectionate pet names of "Dromeda" and "Cissy," but she was often cruel to them.
Bellatrix hated how Andromeda was always going against their family's beliefs. True, Andromeda was Sorted into Slytherin like a good little Pureblood girl ought to be, but she never made friends with any of her fellow Housemates. Instead, she preferred to spend as much time as possible with students from the other three Houses, the inferior Houses, the Houses full of Mudbloods and Blood-Traitors.
The final straw came when Andromeda married a filthy Mudblood who did not deserve to learn how to use the magic that was not rightfully his.
Bellatrix had always hated Muggles, Mudbloods, and Blood-Traitors, but she hated them even more after Andromeda's ultimate betrayal.
Bellatrix hated Narcissa as well, but for other reasons. Unlike Andromeda, Narcissa was always the proper little Pureblood girl. But that was exactly why Bellatrix hated Narcissa so much. Narcissa was Little Miss Perfect.
Indeed, Narcissa was a good girl, but she was also pathetic and weak. She believed in blood purity, but she did not have the stomach to take the Dark Mark. She was not strong like Bellatrix was, and Bellatrix had no problem with often reminding her youngest sister of this.
Bellatrix married Rodolphus Lestrange, but only because he was a proper, respectable Pureblood. Unlike Narcissa, who was always going on and on about how wonderful her precious Lucius Malfoy was, Bellatrix never brought her husband up in conversations unless someone else directly inquired about him. And even then, Bellatrix kept it short and simple.
The Dark Lord was different from anyone else that Bellatrix had ever met before. The Dark Lord encouraged Bellatrix's twisted violence against all who were inferior, and Bellatrix fed on his own sadistic nature.
More importantly, the Dark Lord truly understood and embraced Bellatrix's hatred of Muggles, Mudbloods, and Blood-Traitors. He felt the same exact way about them as she did, and like her, he had no problem with eliminating them. The Dark Lord was strong and powerful. He was not weak like Bellatrix's family was. The Dark Lord was Bellatrix's real family.
Bellatrix gladly tortured and killed in the name of her lord and master, and she never cared how he required her to submit to him. For the Dark Lord, Bellatrix was more than willing to submit. He was magnificent. He was the one person that Bellatrix would never be worthy of, even though she would never stop trying to prove her worth to him.
Bellatrix never understood Narcissa's affections for either her husband or son. Ensuring Pureblood supremacy was more important than family sentiments. But then again, Narcissa had always been too sentimental and too weak for her own good.
The Dark Lord never returned Bellatrix's affections. In fact, he often mocked her for the less desirable members of her family. This, however, did not make Bellatrix angry at him. Instead, it made her even more eager to prove herself to him. Bellatrix gladly killed her treacherous cousin Sirius and the daughter of her treacherous sister Andromeda in order to prove to the Dark Lord that she loved him and him alone.
Bellatrix had only a brief moment to be shocked when Molly Weasley's deadly curse hit her.
As Bellatrix fell, however, she thought about how she had always wanted to die for her lord and master.
But not like this. And not by the hands of a pathetic, sentimental Blood-Traitor.
No, Bellatrix had wanted to die helping the Dark Lord to achieve victory over everyone who was not a proper Pureblood.
But she had failed to do that. She had failed to give the only one she could ever love what he had truly wanted.