I remember that there was another reason for it that Corwin gave, back in Knights of the Tenth World:

> "Listen," he said to Hikaru, who was standing there dumbfounded by his
> bald-faced declaration. "I didn't -want- to kill her, OK? I take no joy in it.
> But I had a choice to make. In that moment, I had her life in my hands, and I
> had yours, and I could only keep one. I had to choose who lived and who died."
> He folded his arms and gave her a steady, calm, rather sad look. "That's what
> it means to be a Chooser of the Slain."

That's probably the one that's more relevant in their day-to-day work. You save who you can, and kill only who you must. While most warriors of good heart and moral standing would agree with that job description, Valkyrie keep it to mind the most, because they know exactly where the souls of those who die on their watch go.