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

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.