LadyLavinia
Scooby
Tara "“ Her death was a complete insult to her character. She was warm, kind, funny, and loved by the audience. She had the sense to get away from everyone else and so remained unscarred by the horrors of S6. But in her death she was not a character, she was reduced to nothing more than a plot device, a trigger for Willow to go evil.
Of course Tara's death was a catalyst for Willow to go evil. That was the point. That is what she was there for. If Seth Green had not decided to leave the show, then Oz would have bit the dust and Willow still would have gone off her rocker.
However, Willow's path to the dark side started long before S6. Probably as far back as S1.