Nature meaning personality and milk meaning maternal warmth. To catch the nearest way means to snatch or grab the first opportunity. In this specific instance, it means the first chance he gets to make sure he will becomes king, even if it means killing someone such as Duncan. The word milk is purposely used as a stereotypical metaphor to give Macbeth a more feminine quality. Mothers who nurse have milk and she is saying that he's more full of that motherly nurturing kindness than she is. Lady Macbeth fear is that the compassionate nature her husband has may keep him from doing whatever is necessary to capture the title and crown of king. As the play goes on she sees that her husband is truly “too full of human kindness” to even think of the plot himself and so she helps.
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