Well, I think the big thing was that Willow was altering Tara's mind, forming their relationship. And they talked about it and Willow promised not to use magick, and then not only does Willow use magick, she uses it to alter Tara's mind... again. I think Tara was right to break up with Will at that point. If the only issue had been the overuse of magick, it would have been different, but once Willow started using that magick on Tara's *mind* there was no way the relationship could have been healthy until Will "quit the juice." It was a betrayal of trust on two levels, and I think sticking with Willow after that would be akin to staying with an abusive partner.