"From the crown to the toe, top-full of direst cruelty"

Lady Macbeth's famous line, "From the crown to the toe, top-full of direst cruelty", illustrates her strong ambition to hoist her husband into the throne of Scotland and to give herself the title of queen. She is willing to slay Duncan in order to achieve her goal. She must be top-full of cruelty so that she can murder without remorse. Mercy causes weakness; if she had any mercy, the king might live and her plot will be ruined.
The concept still holds true today. Most situations in the real world that involve somebody moving into a higher position of power usually requires that person to cause the downfall of their superior. An example is when a politician makes slander about a governor in order to gain more votes for himself in the reelection. They must be cruel to gain the crown.