If we go by the idea that Kumoko was that white loli and she killed Julius, I can only think of strategy. A needed evil to get to a more important goal.
In her loli form, she's not strong enough to overcome the demon lord... or it is not useful to beat the demon lord to save the world. Or she maybe even needs the demon lord as is for future events. They become allies ? even temporarily ?
Also, we do not know the limits of that world. It feels like a video game, but at least some souls/spirits feel real.
Is that some kind of soul upload ? So appart from the classroom souls, everything is virtual, even D's space outside Kumoko's and every other soul's space.
It might be something entirely different I know. But if Kumoko could reach D's space, as has been implied, she probably has to to save the world. If saving the world even has meaning in that story.

On another note, Kumoko is not that strong yet but she set herself quite a lofty goal: save the world, when even survival is difficult.