It seems the princess was simply doing everything that he expected her to do, he was already going to do what she requested of him. But since she didn’t know about that, she’s under the impression that this is a favor to her, she really was quite a crafty one, she could already tell she was probably just dancing in the palm of his hand.
And beyond that, this isn’t just about humanity anymore, all of this will change the very status quo of the world itself. Humans once considered little more than livestock, will become the dominant race, that is if Light doesn’t just decide to exterminate all other races since he said that was also a possibility a while ago. All that matters now is finding the truth, and then from there he’ll decide the fate of the world and the races that inhabit it. Regardless of that however, Humanity can rest assured that will be spared, and more so it will prosper in the new world.
Princess Lilith didn’t make a deal with the Devil, there was no deal to be made. She just did as was expected of her, quite honestly her choices have little consequence, everyone’s choices don’t matter, Light is the one in control, everyone just doesn’t realize it yet. It’s honestly quite scary how much he’s capable of, and how little regard he has for others unless expressly shown otherwise. The only people he sees as “people” are Humans, everyone else is just white noise. It’s both an inversion, and a deep perversion of how things are currently in the world. But whether or not this is for the better, has yet to be seen. And it won’t be until the truth comes out and this “Master” is revealed, until then everything will continue in its current broken state.
Light honestly reminds me of Father Elijah from Fallout New Vegas, wanting to wipe the slate clean and start anew. Except no bomb collars and he doesn’t have a messiah complex.