Everything had gone according to plan. Bellatrix was not by nature the best at following plans, but when she made them, followed them, and they worked out perfectly, it was like a masterpiece—- a work of art by a great master. Only she was the master, in this case.
The plan had been hers from beginning to end. Once she realized that Kreacher was reporting to both Sirius and Narcissa, she knew they could use this to their advantage. She had also suggested to Lord Voldemort that Sirius Black was the key to luring Harry Potter to the Ministry. Lord Voldemort was brilliant and powerful, but he didn't always understand people very well, and he had suggested they use Harry's aunt- Petunia Dursley. Well, Bellatrix knew as well as anyone that just because a person was your mother's sister didn't mean you loved them. Aunt Walburga had always been judgmental and fastidious.
Killing Sirius Black? Well, that hadn't been part of the plan per se, but it certainly hadn't detracted any from her amazing masterpiece. It had been the unexpected pièce de résistance. She had thought it, at the time, the grand finale, but that had been Harry's attempt to crucio her. Really, that had been too much. The Boy Who Never Does Anything Wrong trying to use an Unforgiveable. She could have laughed herself silly!
And now she was doing just that. Even if it hadn't turned out as planned for Lord Voldemort, Bellatrix hadn't been in charge of that part. Her own parts had gone flawlessly. She couldn't have asked for a better day.