How does Macbeth destroy what Duncan built up during his reign? Scotland goes from thriving under a just and caring king to being ruined by a...

When Ross visits Lady Macduff, just prior to her murder, he tells her,

cruel are the times when we are traitors
And do not know ourselves; when we hold rumor
From what we fear, yet know not what we fear,
But float upon a wild and violent sea
Each way and move [...]. (4.2.22-26) 

He means that, during Macbeth's reign, people are accused of treason and have no idea why. In times like these, people believe every terrible rumor but don't even know what to really be afraid of; he says that it feels like being thrown around in the ocean and never actually getting anywhere.  This is what it is like to live in Scotland under Macbeth's reign.

When Lennox is speaking to another Lord in Act 3, scene 6, he says that Macduff has gone to England to meet with Malcolm and

warlike Siward,
That by the help of these—with Him above
To ratify the work—we may again
Give to our tables meat, sleep to our nights,
Free from our feasts and banquets bloody knives,
Do faithful homage and receive free honors.
All which we pine for now.  (3.6.36-42)

Lennox is hopeful that, with England's (and God's) aid and Malcolm on the throne, there will once again be enough food to feed everyone, that Scots will be able to sleep peacefully, that their dinners will be free from violent murders, that they will be able to once again respect and revere their king, and that subjects will again be rewarded for their loyalty and service to the crown. They have none of these while Macbeth rules.