This, a thousands times this!!
There's no point comparing Willow going dark to mental instability.
We literly know what happened, she absorbed the physical dark magic out of all the dark magic books in The Magic Box, she didn't learn all that magic. She physically removed the magic from the books and absorbed it, that magic has physical implications, the Dark magic inside her changed her perspective, it wasn't Willow in control. We know that becasue Giles plan involves Willow draining the good magic out of him in order to give Xander the chance to break through the Dark Magic, presumably Xander couldn't have gotten through to Willow had Giles not done that. There is more at play with Dark Willow then just Willow deciding to be bad.
I'd say she absorbed dark magic much earlier - when she killed the fawn and used her blood for death magic.
This is from the script:
Mmm. Tell me about this spell you
performed.
WILLOW
(suddenly excited)
Okay. First of all - so scary.
Like, the Blair Witch would
have had to watch like this.
(covers eyes)
And this giant snake came out my
mouth and then there was all this
energy crackling and then this pack
of demons interrupted but I totally
kept it together and the next thing
you know ... Buffy.
She proudly awaits his kudos.
GILES
(sadly, quiet)
You're a very stupid girl.
Beat. Willow blinks, re-grouping ...
WILLOW
What? Giles ...
GILES
Do you have any idea what you've
done? The forces you've harnessed?
The lines you've crossed?
WILLOW
I thought you'd be ...
(swallows)
...impressed, or something.
GILES
Oh, don't worry, you've made a
deep impression. Of everyone
here - you were the one I trusted
most to respect the forces of nature.
WILLOW
Are you saying you don't trust me?
GILES
Think what you've done to Buffy.
WILLOW
I brought her back!
GILES
At incredible risk.
WILLOW
Risk? Of what? Making her deader?
GILES
Killing us all. Unleashing a hell on
Earth. Shall I go on?
WILLOW
No! Giles, I did what I had to do.
I did what nobody else could do.
GILES
Oh, there are others in the world
who can do what you did. You
just don't want to meet them.
WILLOW
Okay, probably not - but they're bad
guys. I am not a bad guy. I brought
Buffy back to the world and maybe
the word you should be looking for
is "congratulations."
GILES
Having Buffy back in the world
makes me feel indescribably
wonderful - but I wouldn't
congratulate you if you jumped
off a cliff and happened to survive.
WILLOW
That's not what I did, Giles!
GILES
You were lucky.
WILLOW
I wasn't lucky, I was amazing.
How would you know anyway?
You weren't even there.
GILES
(almost yelling)
If I had been I'd have bloody well
stopped you! The Magicks you
channeled are more primal and
ferocious than you can hope to
understand, and you're lucky to
be alive, you rank, arrogant amateur.
She crossed the line at the beginning of the season and her need for the high that dark magic gave her grew as her life fell apart. In that way it does mimic addiction. But the addiction metaphor falls apart for me once the magic is the weapon - and even further when we are told that quitting wasn't necessary because magic is part of Willow and quitting is impossible. Then it seems like a dammed flood broke loose.
This is where I blame Giles. Once he encouraged and used her for magic then he was responsible to train her. He berated her, but I doubt that did any good. I'd say he seriously dropped the ball.