Disclaimer: I don't own SWTOR.
It starts with a memory. First, the sight of the slight figure of Darth Nox standing before him, her tattoos making her face distinctly predatory. Then, the joy of being chosen as her apprentice. There was something else under that joy, the slight shift in the Force. He had dismissed it at the time. Now he knows better, but with only the memory to work off of, there's nothing he can do, so he dismisses again. For now.
Then comes the words. His eager responses to her questions.
"Tell me acolyte, what would you do for power?"
"I would do anything, my lord."
He was still young and foolish back then, the second son of one of the Dromund Kaas elite. His only thoughts were of besting his brother. He accepts that his words were uttered unthinkingly, and acknowledges that he is wrong. Perhaps that was the shift he felt at the time. Believing he had solved the mystery, he puts the memories back where they belong, in the past.
It's in the middle of the night as he pulls himself out of a nightmare where she's strapped to a torture table screaming his name, and he couldn't do anything because he wasn't there, because he left her behind that it suddenly hits him, that infinite second as he held the last holocron in one hand and the other hovered over the call button on his ship's holo, the thoughts racing through his head, the slight tremor of his finger, the memories of her, of her fire gold hair, of her sea green eyes, of her thrice blasted determination to serve an Empire that didn't care for her, of the brief brush of her fingers against his cheek as she turned to do her duty, and then the all consuming despair as he made his choice, and the bitter numbness that followed him all the way back to Dromund Kaas. Or that was how he remembered it had happened, how he thought it had happened, how he desperately wished it had happened, but the slight whisper of doubt had seeped into his mind.
Was it despair or victory he had felt when he made his decision, had he been numb from the grief of losing her, or the relief of removing his biggest weakness. He tried to quash these poisonous thoughts because it was impossible that he hadn't made that choice for her, he wouldn't believe, he couldn't believe that he'd thrown away his happiness for power, but the words haunted him.
I would do anything, my lord.
I would do anything
anything
And just because he was young and foolish when he said those words, doesn't make them any less true.