Yet do I fear thy nature. It is too full o'th' milk of human kindness

To remember this quote use the following mnemonic.

Yeti, do I fear thy nature?

It is too full of anger. 

I know, I will give it milk with human kindness.

"Yet do I fear thy nature. It is too full o'th' milk of human kindness." Act 1, scene 5, line 15. 

In this quote Lady Macbeth is talking to herself saying that Macbeth is not a murderer by nature. 

Her belief is that his kind nature will prevent him from committing murder.

Milk suggests a motherly, pure and compassionate person.

He has the ambition, but he doesn't possess the type of nature required to take steps to gain the throne. 

Lady Macbeth will see to it that Macbeth gets what has been promised.