None of us will be able to tell you anything for sure. Unfortunately, because we weren't there, out second-hand interpretations of the situation (particular in this anonymous, text-only format) will be biased and limited. As with any romantic relationship/sexual partnership, the situational details, each partner's differing interpretations, and the immense amount of non-verbal communication can drastically change one's understanding of events. It's misleading for anyone to give you definitive answers when they are filling in these gaps for themselves.
However, a few pieces of your story did stand out to me:
She started talking dirty about that, I think it was something like how despicable it was that I was so turned on by that kind of power. Whatever it was, it turned me on more.
...
Then she said she expected me to make it up to her by fucking her hard the next time we saw each other, by giving her the best sex of her life or something like that, like I owed that to her. I don't remember the rest of the letter but I think it was more variations on a theme.
Her dirty talk in bed, and the tone of her letter, seem linked. This is a woman who enjoys dirty talk. In particular, a style that that focused on power dynamics, and addressed those dynamics in a chastising manner, using words like "despicable." What's equally important is that you responded positively to her dirty talk. She saw how you enjoy when she used this type of dirty talk.
From the (limited) perspective of an uninvolved party, it appears that her letter was an attempt to continue this dynamic beyond that night. I imagine that this type of dialogue feels much different when received in writing, the morning after, than it does when directly verbalized, in the heat of passion.
It seems likely to me that she intended her note to be interpreted as an invitation to continue your sexual relationship. However, as the two of you were still nearly strangers, you wouldn't have been familiar with each other's writing styles, and only minimal practice communicating with each other as a whole. OP, you sound like a thoughtful, caring person, and must have understandably been taken-aback the possibility that you could have unintentionally hurt a partner.
It is normal, and, I'd argue, healthy, to have instinctively interpreted the words in this note at face value. When removed from the act itself and the back-and-fourth dynamic of sex, and without the contextualization of non-verbal cues, or even verbal ones, "dirty talk" will often come across as strange, uncomfortable, uncalled for , and hurtful.
Again, I can't tell you anything for certain. However, this unfortunate situation seems to be a combination of two things.
1) She was more into you, and your particular sexual dynamic, than you were. You were fine with your original meeting being a one-time fling, while she made repeated attempts to continue your relationship. She may have interpreted the situation as more of a continuous thing, and, thus, expected her note to be read as part of that sexually-driven continuation.
2) A understandable miscommunication when taking bedroom dirty talk outside of the act itself. She was the one who instigated this style of talk, and while you reacted positively to her dialogue, you seem to imply here that this was something very new to you. This woman my have taken her comfort, and possible experience, with this kind of dirt talk for granted, and assumed that this was something equally familiar to you.
Based on what you have said here, I don't think you have anything to feel guilty about, OP.