Why does Lady Macbeth think Macduff's flight "madness"?
Lady Macduff is the one who calls Macduff's flight madness (Act 4, scene 2). She wants to know what her husband had done to make him flee. Ross tells her to be patient. Lady Macduff responds with
His flight was madness.
She continues to say that when we do nothing wrong, our fears of what may occur make us become "traitors" and flee to other countries. She is upset that he left his family defenseless and that he did not stay and fight to protect his family against what might harm them. She reasons that even the smallest of birds will protect their nest against the predatory owl. For the Macduffs, the owl is Macbeth. This bird imagery continues throughout this act as Macduff upon hearing of his family's slaughter cries out
What, all my pretty chickens and their dam
At one fell swoop
Macduff's fleeing, Lady Macduff declares, goes against reason and is motivated by fear.
Ross tries to defend Macduff by assuring Lady Macduff that her husband is
noble, wise, judicious and best knows
The fits o'th'seasons
He reminds her that times are cruel and that innocent men are considered traitors, making these men fearful. He continues to explain that things will hopefully get better. In this chaotic world that Macbeth has created, good men become traitors, brave men must flee, and innocent women and children are slain.