I do think Willow's reasoning was dampened due to the shock and grief. The dark magic she sucked up added to this condition. That doesn't mean she was evil or stopped to care about Buffy. Call it temporary insanity. After she shed Warren's blood she didn't felt the satisfaction she expected or needed, so she went after Andrew and Jonathan. Since they weren't responsible for Tara's death, you could say she wasn't in her right mind anymore.
After she drained Rack she lost her mind completely. The wanting to destroy the world thing is partly Giles' fault. He intended for her to suck up his borrowed white magic. A risky strategy which had backfired if it weren't for Xander. Willow was absolutely not sane anymore. Unfit to plead is the law-term if I'm not wrong?
If there had been a trial which is competent, it would have declared her not criminal responsible due to the circumstances. She'd been most likely put into preventive custody with much, much counseling until she isn't a danger for society anymore. In addition, she's found guilty of gross negligence with assault and battery (Tara & Dawn) as result for her flippancy with magic, taking mitigating circumstances (addiction) into account. Probably a probation.😀
Somebody firm in law here? Is that somewhat accurate? To my defense, I rely on my amateur knowledge in German law here.😀
And I just realized that I've got completely off-topic here. Somebody interested in a new thread about questionable deeds of all characters if the supernatural had been a part in the human system of justice?
On-topic: A further honorable mention of heartbreaking deaths is the deactivation/destruction of April and Buffybot. They were quite lovely IMO and their respective ends were dolefully put on screen.
And Tara might died quick but the people around her really suffered.