It makes a mockery of his entire lived experience and it pisses him off and he feels the desire for vengeance intrinsic to human nature that he has rationally analyzed more than once over the course of the movie. He can convince himself that Kutcher's character is completely right because it dovetails with his own personal philosophy of being a detached observer and that is, at that moment in time, too much of an injustice to bear because this is the most he has cared about anything in his entire life.
The meaning is literally in the title of the movie.
