Why did the horned woman leave Vayne alive that night? (Potential Darkin Lore?)

I mean, you don't leave witnesses after a crime and even when you are a sadist you kill them after you've shown them your bloody masterpiece. No, this was done purely intentional, indicating that the horned woman has a plan for Shauna Vayne.

And she is also described as a demon. And what do demons do? They CORRUPT people. And when I look at Vayne who is a bloodthristy and sadistic killing-machine with a burning harted to all she even remotely suspects to be magical and who finds ENJOYMENT in the slaughter... well, it'd say the corrpution was a full success. She even killed her own adoptive mother and teacher without a second thought! It can't get many more obvious than this I think.

But was this "beautiful" horned woman truly a demon? Or maybe something else? There exists one other entity that influences and "inspires" humans to the point where they become brutal killing machines beyond imagination. Obviously, I'm talking about Aatrox here. A small faction of soldiers losing a war and traumatized and scared in the face of death and suddenly they win by just slaughtering everyone in blind rage? It is like someone utilitzed their suffering to change them, for what puropse I can't say - and the same might have happened in a smaller scope with Vayne.

Maybe different Darkin operate differently? One choses armies, the other picks a single "champion". But... why? Just what are the Darkin scheming? Or are they even working together? Or maybe even against each other? We clearly lack lore in this regard...

On another note I wonder how Vayne would react when she realizes that her entire life she had been the pawn of the one she hates with passion - and that this hate was completly predicted and planned? It's like realizing your entire life has been a lie - but after reading what she did to Frey I am not sure if I should even have pity with her...