I like Dark Willow. She was so evil!
If she has the same deep personality traits/flaws of moral relativism and emotional immaturity/inability to handle negative emotions healthily then absolutely.
I would want more serious consequences for her actions though so she could complete some kind of coherent character arc.
The trick is if you're not dealing with moral relativisim you're dealing with moral absolutism and it's hard to pull off shades of grey with that
But then you have the problem of they take place in the same universe so shouldn't the morality be the same?There are degrees. Buffy killing 99.99% of all vampires she encounters on the show is pretty absolutist and yet vampires and slaying are not depicted as boring. And too much moral relativism is what the show itself contracted in S4 and it got worse as the show continued.
Generally speaking Angel's show was better at that kind of thing, partially because Angel himself was just more morally compromised from the stastart
But then you have the problem of they take place in the same universe so shouldn't the morality be the same?