One other scenario is possible that hasn't been mentioned. An RPG can be heavily focused on violence, but be strongly skewed away from death and murder. An example would be 'Mutants and Masterminds' played under its default rules. M&M emulates a comic book universe, so the expectation is for large amounts of violent conflict, that usually end up only with heroes or villains merely temporarily defeated or incapacitated.
On the broeader question of, 'Why the killing?', the most likely answer in my opinion is because role playing sex with each other makes the players of the game uncomfortable. That's only somewhat tongue in cheek.
Violence is strongly linked to pleasurable responces in humans, especially males but even in females as well. A game with a large amount of violent conflict helps keep the players from being bored. Also, violence can be intellectually as well as emotionally stimulating. It's not just role-playing games where the focus is on violence - virtually every board game features some sort of abstract violence. The whole notion of 'tactics' is intimately tied to violence. Most sports feature some sort of controlled (or not so controlled) violence, and even those that don't usually rely on displaying and honing the skills humans employ to commit violence. So with violence you have an easy mentally stimulating jolt to the game session. It's used in movies for much the same purpose. Even a bad movie can keep many people's attention if it has enough violence in it.
Violence is strongly linked in people's minds to the scale of the conflict. If the matter isn't worth risking violence over, then chances are people percieve the matter as being petty or trivial. If the matter is worth investing emotion in, then its percieved instinctually as something worth getting angry over. If its percieved as worth being angry over, at some point people will assert the rightness of violence. People will talk about their 'right to be angry' and the 'justness' of their violent reaction. And even if you don't agree with that, that in itself is an interesting emotional conflict and moral philosophy that only can be explored in the context of violence.
Pacifism is a very rare moral position, even among pacifists. While quite a few people claim to be pacifists, in practice virtually no one actually practices it because the logical upshot of pacifism is that someone who isn't a pacifist kills you in very short order. One really easy test of this is to ask the supposed pacifist whether they believe that they should contact the police in the event that they have been wronged (robbed, assaulted, raped, had a family member murdered, ect.) If the answer is yes, then they believe in 'pacifism for me, but not for you' which is far far more common of a position than actual pacifism. Likewise, ask yourself if the pacifist community has put itself in a position where its existance depends implicitly or even explicitly on non-pacifists protecting it. An example of this would be various caste systems, whether in medevial europe or India where the high caste 'pacifists' depend on 'lesser honorable' castes for protection and legal enforcement. In any event, the upshot of this is regardless of the focus of the game, if the topics explored in it are serious enough, the game will probably eventually feature a choice between violence and non-violence and most players will probably at some point regardless of their beliefs feel compelled to resort to violence.
My little ones are natural RPers. They'll sit and play informal roleplaying games with each other for hours. Usually what they are playing bores me to tears. I can't play with them for more than 5 or 10 minutes because the scale of the conflict is so petty. They RP 'where they are at' regardless of setting, and so conflict in their games revolves around whether or not a character is using good table manners, whether or not a character is sharing the toys, and who gets to take a turn at some occupation. The extent of the violence being played usually quite appropriately to the scale involves nothing more than scolding the naughty character. They play what ammounts to a rotating GM structure, and after a little coaching quickly grasped proposition/response structure, and the separation that prevents a GM from taking control of another player's primary character (afterwhich the number of metagame conflicts drastically decreased). As far as I can tell they play in an unstructured improvisational sandbox with no metagoals other than absorbing themselves in the moment. It's quite engrossing to them, but not particularly engrossing to me yet (though its getting there).
However, even they don't play a game that is fully without violence and killing. Sometimes naughty characters break the rules and find themselves killed. Sometimes naughty characters commit murder and have to be hunted down and put in jail. Some of the heroes are effectively 'anti-heroes' who are swift to punish naughty characters with instant death. This doesn't serve to make the game any more interesting for me, but does suggest that as there game play evolves into more mature games and as they start to find the need (or at least intellectual ability) for more formal game structures that the resulting games will feature at least some violance and killing.