That's actually a good question, and I shall answer it. Spirits aren't corporeal beings, and therefore go by different rules. The tapping of energies, especially those regarded as sinister or unwholesome, is actually a good thing. Think of it like trimming a bush or tree to help the whole plant, even though it's nothing at all like that.
And yes, those who can ask not to be eaten should not. But I wasn't about to leave a dangerous spirit, however contained, in the area. I've nothing against spirits, even those harmful to life, but that spirit would have been a problem if I'd let it live.
Arbitrary? Perhaps. I'd grown more wary of spirits since adopting my first. I might still have fed that young shadow spirit, but I don't think I'd have let it in my aura.
Yuck.
I needed to clean my aura.