"SILENCE!" the King of the Turtles barked back. "I'm King, and you're only a turtle named Mack."
This quote is important in that it establishes a hierarchy of importance. Yertle is King and this immediately makes him worth more than a simple turtle like Mack. His opinion of himself is so high that he forgets he too is also 'only a turtle'. The fact that Yertle distinguishes himself from the rest of his kingdom is stressing the overarching social class system that divides the normal people from the elite.
"He made each turtle stand on another one's back
And he piled them all up in a nine-turtle stack."
This image of the turtles being stacked up against each other of highly disturbing. Older readers will note that Yertle is meant to be an allegory for Hitler. The pile of turtles can therefore be a reference to the piles of bodies that Hitler left behind in his rise to power. His power was built on by the very people who he goes on to abuse, much like Yertle.
