It sounds like you met a genuine psychopath. I was on the fence until you described how you met and how she made a gift for you and pledged her friendship to you later in the week you met her. That everyone’s response to your concerns she would turn violent was surprise, and that she walked away unscathed without a desire to get revenge is to be expected from a psychopath. As for the ‘why,’ it’s almost never fruitful to even ponder in the case of a psychopath. They choose a goal. They meet the goal. There is nothing more. Sometimes the goal is to become a respected member of society. Sometimes it’s to get rich. Sometimes it’s to do nothing and have someone else take care of them.
Sometimes it’s to feel something. And that is when they are at their most dangerous.