The exact wording is "Should I fall, I have one last command you would be wise to fulfill.  Kill Molly Carpenter.  As quickly as possible."  Harry asks why and Mab explains "As Winter Lady she shows promise.  But she is not ready to become Mab.  The consequences would be unsettling....for both of you.  Perhaps for all of winter"

To me this goes in direct violation to the deal where Harry said "And you give me your word that you will never command me to lift my hand against those I love"

In one statement you have at least the implied agreement from Mab that she would never command Harry to harm those he loves otherwise he would not have accepted the mantle.  In the other statement she is commanding him to kill someone he loves.