I don't think it's actually clear whether or not that Willow was bad, though. I believe she had to use dark power to sustain her until Buffy arrived, but we haven't found out her full intent yet, or why she ever decided she had to do that in the first place. Hoping they explain that soon...If you prefer to just hear about it rather than reading the comics...
She's not bad now, but she was in the future.
(In the "Time of Your Life"story Buffy goes 200 years in the future where Dark Willow is waiting for her.)