Also, sorry about the fridge reference. It wasn't entirely a correct trope, as it was closer to this
one, but really what I meant was that they killed a character (a female character) to advance a main character storyline. Named after the website Women in refrigerators.
I'd argue that, by the definitions on TVTropes, it's much closer to "Stuffed into the Fridge" than "I Let Gwen Stacy Die"; the latter is characterized as a character death which raises the stakes (or makes them real) for the hero, while the former is a specialized form which is usually brutal, unexpected (yep), and directed at a female character the writers never figured out how to use properly (yep). Plus, Peter Parker didn't fly into a horrific rage and murder Norman Osborne. Tara's death didn't raise the stakes; they completely altered what was going on, simultaneously making the villains much, much worse (all part of the brilliant deconstruction of supervillainy that was "the Trio") and changing the essential problem of the season from them to Willow.
But yeah, I was never a fan of what Joss did with Tara, and killing her just bugged the hell out of me, because, while she really needed to leave the cast, they didn't need to kill her either, especially as a plot device.
Not to be contentious or anything, but I disagree! Her death is emotionally painful, thematically resonant, and a wonderfully effective plot twist. They used every bit of the goodwill they had built up by leaving Tara out of conflicts and put it to incredible effect. Forget about what it did for the season, or for Willow's character (or for the show); nothing else they could have done with Tara's removal from the cast would have been anywhere near as effective, let alone in the multiple areas that it was. The only two possibilities consistent with her character up to that point were "Tara leaves Sunnydale to get away from/help Willow" or "Tara goes back to her family now that Willow no longer anchors her" and neither of those have the thematic (much less dramatic) weight of what happened. Again, I wish we had gotten a useful Tara over the course of the series, but if she had to go, this was the best way to do it.