Not sure about that... I mean, even if Willow had gone dark by overdosing magic, those who liked her would still do since pretty much every character goes through an 'evil' phase at some point. I might be wrong, but a character going dark doesn't mean peoples stop loving it.
You're right, if a person is really dedicated to a character they'll continue to be even if the character goes "evil", but, I meant that Tara's death allows Willow's actions to be justified. Like an excuse.
The writers had spent the entire season establishing (or continuing) Willow's descent into villainy, so she was going to go evil. Nobody needed to die for Willow to go dark. She was doing just fine on her own. But, I think that if the writers would've just had Willow start randomly killing off jerks like Warren people wouldn't approve. He's a jerk, but that doesn't mean he deserved to die. But, having Willow go crazy after Tara's death and dole out "justice" to the murderer makes people go "Well, it's okay because Warren's a murderer." Even though, it's not okay.
I don't know, I'm still unsure about that. One of the reasons Willow wanted to change and "be clean" of magic was because she loved Tara. After they broke up and she almost killed Dawn, Willow sought to get better. She even refused to do magic in "Older and Far Away" when they were all trapped, and Tara was very proud of her for that.
Willow stopped doing magic, but there was no way she was going to keep it up. If you look at Willow's storyline as magic = drugs, then Willow's not going to kick her habit by giving it up cold turkey. Most people can't quit drugs just because they want to, they have to be physically and emotionally weaned off them. If they don't, they could relapse, bad. Such was the case with Willow. As much as the writers wanted to show that Willow was doing the right thing and kicking her habit, she was going to relapse. Tara's death did it. But, it didn't have to be Tara's death. Any emotional issue, big or small, would've caused Willow to retreat back to comfort of magic/drugs.
And, if you look at Willow's storyline as magic abuse being a metaphor for abuse of power, Willow wasn't going to be able to keep that up, either. For Willow to stop abusing magic she would have to realize she was doing it and why. Only by learning
why Willow does the things she does would she ever deal with the problem. During S6, Willow realized she was abusing magic, but not the reason why. She only stopped using magic so her friends and her ex-girlfriend would continue to like her. She didn't stop because she thought it was wrong to raise people from the dead or violate your girlfriend---Willow's never expressed that she thought any of those things were actually wrong. And, just because Willow's stopped doing magic doesn't mean she's stopped abusing power. Because she doesn't realize what she's doing is wrong, she would have probably found a non-magical way to do it.