Milk of Human Kindness

[insider]

Where does this phrase come from? What exactly does it mean?

[Fretful_Porpentine]

Macbeth:

… yet do I fear thy nature;
It is too full o’ the milk of human kindness
To catch the nearest way: thou wouldst be great;
Art not without ambition, but without
The illness should attend it: what thou wouldst highly,
That wouldst thou holily…

In other words, it’s the stuff that keeps most of us from bumping off everybody who gets in our way.