All characters belong to J. K. Rowling. The circumstances are created by me. This tale is also very loosely based on "The Imp of the Perverse" by Edgar Allen Poe. They'd probably kill me if they read their ideas being utilized in this way, but oh well.

Solution

The silver moonlight gleamed off every strand of her hair, making its midnight black shine like an unfathomable river running somewhere yet unfound. He appreciated this last moment of her beauty as deeply as he could, and impressed it forever in his memory. She had been loyal, and served him to the very extent of her abilities. He had never truly adored her but he kept her close in relationship and was content.

Then, she had changed.

The changes were very subtle, but he had caught them with his keen eye and mind. Blessed with a husband as she was, she began to spend more time in his presence, and he had seen how she tried to hide it behind business matters and duty callings, so insecure, so afraid of discovery. He had never told her that he noticed. Instead, he had watched, waited, and planned. He had sensed her looks directed at him, sometimes a glance, sometimes a vacant stare, and he had turned away from them. She was beautiful before the hiatus, that he could not deny, and being years out of prison, she had mostly regained her beauty. The trouble was, sometimes he could not help looking back. Never have he met someone who was so startlingly similar to himself in personality, except she vied only to serve, and that was fine with him. Still he had waited.

As the days passed, he could almost see her mind working, trying to figure out her way around the obstacle. Then one morning, he had noticed another change, this one bringing out a feeling of steely nerves mixed with a sort of anxious dread and desperation in her. He knew, and he followed. The basement of the mansion was converted into potions workshops, and he saw her descend the stairs the same way she would soon descend to her doom. He had simply stood in the adjacent room as she worked on her potion, and listened to it bubbling and sizzling with astounding patience. He had glimpsed her passing, carrying a vial intended for someone's cup. Things were then solidified, and he would not have stopped her in her mission. By then, his resolve had taken hold, and he could not have stopped the events that must follow from unfolding.

The next morning, he knew even before she had told him that a Death Eater had perished. He had not taken the news as a serious matter, and felt her joy and feeling of fulfillment at his response. He knew the time was soon, and anticipated the moment of his fulfillment.

For that night, she came.

He was standing by the window, with the moonlight flowing through and casting his shadow on the floor. Just when he had expected it, the double doors eased open with a small squeak. He could hear footsteps, purposely slow and made very quiet, almost like shadows of sound. It was undoubtedly her. He did not turn, and felt her confidence somehow increase as she approached the silhouette in the wide window. At about ten paces, she halted, and remained standing at that respectful distance, as if needing a prompting. It was then that he decided to turn around, and so he did, slowly on one heel, just to make her more fearful and even more anxious.

Through all of his planning, he would not have prepared himself for what happened next. As if bidden by some unseen supreme being, she sprang to action almost too fast for his eyes to see, and ran the last few paces between them. She stopped just as suddenly as she started, and them embraced him. He put one arm around her shoulders and the other around her waist.

Even as they stood like statues, sprayed with moonlight, and even as he lost himself in her physical warmth, he was thinking of what he was about to do. For a fleeting instant, he thought that he could live like this, with her by his side, but in his heart he knew that was not even a possibility. He would never be able to give in return what she gave and will give him, and it pained him that he could not. Therefore, as he had previously so carefully figured out, one way remained.

His right hand went to his pocket, and he even held his breath. She might or might not have sensed something amiss, but at that instant she drew back, and almost stepped back a pace, as if horrified at what her impulses have led her to do. With the speed of a viper's strike, he drew from his pocket not his wand, but a honed silver dagger, its deadly blade shining in the moonlight. With another swift movement he drove the blade hilt-deep into her chest.

She might have screamed. He was positive that she would, he would even enjoy it if she did, but he had his own impulses. He did not know what drove him to it, but the moment she opened her mouth slightly, he tilted her chin up with his other hand and kissed her, giving the last few moments of her life his long-awaited blessing.

They held this position again, until he let go and her body crumpled to the floor, bleeding onto the wooden boards, the dagger still shining like a star that brings death on its arrival. He stayed where he was, examining his work, lost in the thrill and the confusion of what could not have been more than ten minutes, and yet was the result of many lessons in his life, and an enormous lesson in itself.

He knew that it had to come down to this, because he could not have loved her back. There was no other way.