I am not entirely certain that there will be a direct quote, word for word to reflect the cruelty of reality.  Steinbeck presents characters in the novel who do not ruminate on being in such a manner.  You might be able to find particular moments, quotes that reflect instances where cruelty is a part of what it means to be a human being.  These moments can be seen in the discussion where Candy's dog is planning to be executed, and the cruelty in Carlson's sweet voice to lure the dog to his own death.  Cruelty is evident in the conversation between Curley's wife, Crooks, and Candy.  At a moment when there is hope, there is the idea of collectivity and unity, Curley's wife undercuts it with venomous cruelty.  Consider also the ending of the novel itself.  When George shoots Lennie, it is a moment where there is care as George does not want to see Lennie at the hands of the mob that is coming for him.  Yet, the act itself is cruel, regardless of how one tries to spin it.  The taking of another life is a cruel action, and this is something that George knows, needing to be steadied after it.  When Slim tells him that George "did what he had to do," it is a reflection of how cruel the world can be and how much the power of rationalization plays in our actions that reflect such cruelty.