Willowgreen's trickery-why the Go-Backs have no (or very little) wolf blood?

My theory is that she did it out of love. There is evidence that it was Two Spear's wolf blood that drove him crazy (see Kahvi #2 and #3) and she wanted to perhaps give Two Spear and the other Wolfriders "peace" by taking away their wolf blood. Another thought is that Graywolf having a lot of wolf blood might have been aging much more rapidly than Willowgreen and she might have clensed the Wolfriders to extend Graywolf's life. Willowgreen was wrong in what she did because it was apparantly non consensual, but it didn't match the evil of Winnowill. If Winnowill was a good, though bigoted elf, she would have cleansed the Wolfriders, willing or not, rather than try to kill them.

Very likely she either slowly sapped the wolfblood out of them over a long period of time, or somehow managed to do it in thier sleep before vanishing away. As to her motives, it does seem likely she did it to try and cure them of the issues the wolf blood seemed to inflict upon that group... but its also entirely possible Two-Spear was crazy anyhow and simply killed her in retribution for taking the wolf out of him, and I wouldn't doubt that possibility there.