The desert's a killer. It takes everything from you in the end.
It was so fresh for Ahsoka because it hadn't been very long ago he'd said that, not wanting to talk about it, but she unable to just ignore that hurt. It was more than she would have imagined, the reasons behind it, but that also took a second place to his continued explanations. She'd imagine all he'd said thus far were the most shocking things she could be told, but the shock and disbelief and overwhelming confusion continued.
Master Windu was gone, murdered by Palpatine and-
There was a childish part of her that wanted to say, no, not my Master, before processing the 'who' but it was only a want. It wasn't even a fully formed thought because everything else argued the point.
Darth Vader.
Anakin Skywalker had become a Sith Lord. The Sith were capable of many great powers, and like the Jedi, capable of concealing themselves within the Force. It was obviously how the Chancellor had fooled everyone - and it had to have been everyone, as he would never have been allowed to live and control the Republic otherwise.
But Anakin wasn't. He wasn't concealed from her, at all, and she couldn't believe all the things she could sense in the Force around him, so clear and raw that it might as well have been a holo playing off to the side of him of all the things he'd seen, felt and done, were fabrication. He wasn't lying and he wasn't fabricating anything in order to accomplish some goal – not killing her, she wouldn't think that – and even had loyalty not stayed her hand by default, all her senses did.
Anakin Skywalker had become Darth Vader, but he wasn't Darth Vader now. She knew that, even if logic dictated she had now skated entirely around aloof objectiveness and straight into messy personal bias that made her judgment more than suspect. That change in him, that indefinable Dark that lingered, yet did not taint, was suddenly given an explanation.
All she could do was stare at him, wide-eyed, and wait for the next shock to come. She couldn't imagine anything from this point would be anything but even more impossible to take passively.