Angst? What Angst?: Justified by Slevin with his "ataraxia". The true reason is of course that Slevin planned all the events of the movie in advance.
Although one would think it would be easier to fake being scared than fake a lack of feelings with a gun to one's head. He's also pretty quiet for a guy who is facing off with the men who murdered his parents, and against whom he has orchestrated an elaborate revenge plot. So its likely he DOES have a certain numbness toward his emotions, like his mentor Mr. Goodkat.
Funny that he presents "ataraxia" as a condition; it's a Greek philosophical concept and considered something to strive for by various Greek schools, such as the Stoics. Which suggests he has been trained to suppress his emotions.
Critical Dissonance: Most critics hated the film, or at least gave it mixed reviews (at the time of writing, it has around 51% on Rotten Tomatoes), but it's generally well liked by the regular public- it has an 85% score on Rotten Tomatoes from fans.