Except that she slaughthered Forsakens when they showed free will, created skeletons which served mindlessly her at the Undercity battle and expect the same level of blind loyalty that Blue Man.
But i agree, this action is in lieu with what we know about Sylv. She is dumb idiot who cant foresee her actions consequences above 5 secs tops. Started with power slide into Arthas and was trainwreck from here on.