Not Voss, but are there no death knights in the Alliance? There are. Or are they "nice" Death Knights? They don't raise mindless undead cause that would be... evil. Voss is simply a character who accepted her situation and puts her experience to use. In fact just today (spoiler) I watched her try to help a former kul tiran commander accept her new ... life as a Forsaken. It was NICE!
It's amusing how certain people turn against any character that supports Sylvanas. Almost looks like irrational bias.