To open up(To begin), what catches the eye at first is when the sniper's physical appearance is brought up to the table: «His face was the face of a student [...] but his eyes had the cold gleam of the fanatic [...] the eyes of a man who is used to looking at death.» (l.11-13) In the same person it is possible to perceive (both) the youth and the innocence of a student and the strong, silent composure of a killer. It is hard to see someone who physically looks like he has all his life in front of him having an ease to kill. If the physic doesn't match with the look, that may lead to discover someone who is older mentally and emotionally. contrast
To go on(To continue), the sniper deals with mixed emotions when it came(comes) down to shooting his rival. At first «The Republican sniper smiled» because he knew he was about to kill someone who had no idea of what would happen, and then «uttered a cry of joy» when the death seemed confirmed. On an other hand, the emotions suddenly tend to overturn and be