That sounds like the rationale of a psychopath trying to convince himself that he's not really evil. Sure, you could just use it to kill Bandits in self defense and then turn around to use the souls only to make "Good" artifacts, but that would be simply an attempt to redeem yourself for the inherent Evil. It's a tool, not a weapon, but it's purpose is to steal souls for power. Black souls, intelligent people presumably with lives, hopes, and loved ones. (If the AI wasn't so shallow.) This isn't a grey area, it's darkest matte black, like a singularity sucking up all around it so that it can become more massive, and dominate an ever greater radius. It's even color coded for your convenience. The mere fact that you're betraying a Daedra (You openly admit isn't purely evil) and her priestess doesn't nullify that, especially when you count all the people who you have to betray, and prevent from reaching their respective afterlives (Most of them Nords, so Sovngard) so you can become a more effective killer. That's not even a slippery slope, it's diving into the abyss with such a quickness you approach the light barrier.