My name is J.C. Denton.
Well, no, it isn't. I am Peter Rauch playing Ion Storm's Deus Ex, and even diegetically—that is, even from the perspective of the game's internal world—J.C. Denton is a codename. As Denton, I am infiltrating the ruins of the Statue of Liberty, which have been occupied by a terrorist group called the NSF. (The Statue of Liberty is in ruins because a different terrorist group has blown it up several years earlier. This attack on a major American landmark has allowed the government to launch a global war against vaguely defined “terrorists” and clamp down on civil liberties in general. There is much to be said about this plotline; I will say only that it sure seemed like a fun escapist fantasy back in 2000.)
My diegetic brother, Paul Denton, is assisting me on this mission. He reminds me that I am serving in a police capacity, not as a soldier, and encourages me to minimize bloodshed. I am armed with a 9mm semiautomatic and a short-range stun-gun, and Paul allows me to choose a third weapon. If I select the non-lethal tranquilizer crossbow, Paul is pleased; if I instead opt for the sniper rifle, he is concerned, asking that I remember that I'll be shooting at human beings, not targets.
Every character, in fact, seems concerned with my attitude toward the casual application of lethal force. Only Paul seems opposed to it. In fact, if I kill too few people, and gain the admiration of Paul, my other comrades will doubt my commitment to the mission. Two opposing viewpoints on the morality of my killing are clearly established. Taking actions that satisfy either viewpoint will please some and displease others. My own beliefs concerning the morality of violence color the proceedings, of course, and I therefore consider one path preferable to the other. However, from my perspective as a player, and not as a character in the world of Deus Ex, the two viewpoints are distinguished differently. From a purely practical standpoint, completing any given part of the game with a high body count is much easier than doing so with a low one.
Deus Ex has only three non-lethal weapons, and they all require more skill to use effectively than their lethal counterparts. As the game goes on and my foes become more difficult, this skill difference becomes greater, and one might expect that the treatments of lethal and non-lethal violence would become more disparate.
This is not what happens. At the end of what could be considered the game's “first act,” Paul reveals that he has been working for the NSF all along. It is never made clear if he opposed the gratuitous killing of NSF agents because they were human beings, or because he was secretly on their side. This plot development could be read as an endorsement of the “mercy equals betrayal” attitude espoused by J.C.'s more bloodthirsty comrades. From this point on, while the game itself continues to make distinctions between “dead” and “unconscious,” the characters in the game do not. Characters drugged into unconsciousness are treated by other characters as being dead. At this point, combat functions much like any FPS: if something attacks you, empty as much of its blood as possible onto the floor.
In Deus Ex, the reasons we do not generally engage in wanton homicide in the “real” world generally do not apply. Beyond some vaguely-realistic faces and voices, the NPCs in Deus Ex are not very much like human beings. Whether he leaves them conscious, unconscious, or dead, J.C. rarely encounters any specific enemy more than once. The gun-toting NPCs are, on one level, problems to be solved, and it so happens that the sniper rifle is much more effective for solving problems than the crossbow. So why would anyone want to use the crossbow?
One reason, of course, is because the crossbow is less effective. Non-lethal weapons require more skill, but developing and displaying skill is one of the things that makes videogames enjoyable. Variety is another reason, as players tend to seek out multiple ways to play a given scenario. Players who apply a role-playing element to the game might opt for non-lethal tactics because they wish to impute their own morality onto J.C. For this last reason to function, however, another more fundamental reason must already be in place. Why would players want to minimize NSF casualties in the face of greater difficulty?
Because the game will notice if they do.