She murdered millions -- potentially billions -- of people. What the fuck difference does it make what world she's from?
All of it. In this make believe world she's from, the moral laws of that world are to do unto others before it's done to you. There were no 10 Commandments like ours (whether you believe in them or not). There was no Sesame Street teaching about sharing. Or annoying purple dinosaurs to say to love one another. The opposite occurred. In this world, everyone born is taught to fight. Everyone. That's all they know and it's accepted by everyone that this is how it is. If you, personally, were born there, you would grow to be the same way. I would. We all would. The evolution of that planet is one where there is no moral compass like ours. And it's not the fault of the person born on that world that their upbringing and culture distorted it so. How do they know any better when there's nothing else to compare to?
The fun part about science fiction is you get to take that idea and then say, well, what if someone that was literally programmed on that world to not know any better was suddenly thrust into a scenario where they were shown that their whole upbringing and beliefs that were instilled upon them since birth were completely wrong? Watching the choices they then make is the interesting part.
But trying to compare that person to someone who is evil in our real history despite being brought up in a culture that overall believes in the value in life and others, that's a different beast. You can't compare the two. The societies that constructed morality and laws in which they were raised are opposite from each other.
On top of that. The people that she killed, as far as the main timeline/universe are concerned, don't even exist. There are zero bodies. Zero graves. The people she murdered are happily living their lives and doing their thing in this universe. Can a judge or jury condemn someone if there isn't proof or evidence of a crime? And all evidence actually points to the opposite?