Oh, yeah, she's clearly unstable from the moment we meet her. Even better, she chooses to be unstable. In her very first appearance, she calls on supernatural forces to remove all traces of compassion:
Come, you spirits
That tend on mortal thoughts, unsex me here,
And fill me from the crown to the toe top-full
Of direst cruelty! make thick my blood;
Stop up the access and passage to remorse,
That no compunctious visitings of nature
Shake my fell purpose, nor keep peace between
The effect and it! Come to my woman's breasts,
And take my milk for gall, you murdering ministers,
Wherever in your sightless substances
You wait on nature's mischief!
She knows that she has compassion, and wants it gone. That presages both her viciousness and her breakdown over the consequences. This isn't the speech of somebody entirely in her right mind, right from the start.
There's another particularly horrifying sequence:
I have given suck, and know
How tender 'tis to love the babe that milks me:
I would, while it was smiling in my face,
Have pluck'd my nipple from his boneless gums,
And dash'd the brains out
There's a huge amount to be made of this. There are no other references to the Macs having children, and in particular no sign that Mac was planning to leave the throne to his family. She must have lost a child: a common enough occurrence at the time, but don't be misled into thinking that they were unaffected. In performance, there's a huge well of grief to be had there.
I want to point out one other line, kind of unassuming, but highly effective in performance:
Things without all remedy
Should be without regard: what's done is done.
In context it's very easy to see that as simply callous, telling Mac to man up. But there's an undertone that says, "OK, maybe we screwed up, but it's finished and we can't fix that so there's no point in feeling guilty".
That is the strongest presage of the mad scene: she does feel the guilt, just as she knew she would. It's reflected in her final line:
To bed, to bed! there's knocking at the gate:
come, come, come, come, give me your hand. What's
done cannot be undone.--To bed, to bed, to bed!
She's re-living the moment (thus, the knocking at the gate, introducing the Porter scene). And she repeats her line, this time desperate to forget. She cannot, and soon she will be dead.
