For what it's worth, my first impulse was to think that her mother was the one she wanted to kill all along -- that her minders, Konstantin and Dasha, channeled and then honed that energy, and that crazy, to their own ends. And having achieved her ultimate goal, however subconscious, Villanelle's heart isn't in it anymore. She can definitely still do it, she just no longer takes pleasure in it. As you alluded to, she has been heading towards this since the 1st season, eternally dissatisfied. Now it's time to find something new, and maybe that something new is caring for someone else.