There's two concerns here.

One, video games have rating boards (ESRB in North America, PEGI in Europe). Why? Because people understood that games are just like any other type of entertainment, and can be enjoyed by a large population. Like other types of entertainment, games allow the creators to tell stories, pass messages, tackle life issues, and touch on subjects that are not appropriate for everyone. That's why some games are made for kids, others for adults. Violence in games will limit the rating: 13+ for implied, cartoonish and otherwise non-gory violence, 17+ for realistic, sometimes set in modern times violence, and 18+ for absolutely gruesome and quite extreme instances of it. Look at the rating for It Girl: 4+. That's right, the game is specifically targeted at pre-teens, especially when you look at the flashy colors and "prettiness" of it all. I'll take the opportunity here to mention one game that I was at first surprised received a 17+ rating: The Longest Journey. For those who haven't played it, it doesn't contain violence, doesn't contain sex, but at one point treats of mature themes of a sexual love triangle, boyfriend issues, career and life prospects and parental relationships in the life of young adults. They just talk about it, but the themes are mature enough to warrant the classification, because a teenage boy or girl might not be emotionally developed enough to understand what's going on. The point I'm making is: games are made for an audience, and mature textual elements are just as important as visual ones.

Two, it's completely faulty to equate fantasy role-playing and realistic role-playing. Let me illustrate: how would the world react to a game where boys are invited to hit their girlfriend? Quite badly I'm sure, yet purely objectively, you could argue that killing someone (even an enemy combatant) is morally more reprehensible than injuring a person. So why is the former unthinkable, yet the later is ok? Before we someone starts talking about "decline of morals", I'll offer another examples: children stories. Who has been told bedtime stories in which a witch burns kids alive in a stove, princes fall to their death trying to rescue a princess, and a mermaid not only sacrifices her ability to talk, bears excruciating pain when she walks, and arguably commits suicide? And we tell these stories to kids aged 5, yet they don't become apathetic, quite the contrary, these stories motivate and develop human emotions in kids. What's going on? It turns out we can demonstrate kids, even very young, can correctly separate reality from fiction, and fiction is a way to release anxiety, fears and desires without affecting the real world. If it wasn't, then we should be putting people who went to the theater to watch Saw behind bars. Look at It Girl: it's not rooted in fantasy, it's very clearly supposed to reflect the real word, in a way Mafia Wars doesn't (btw, the latter is rated 12+). If mafia wars was about bullying people in high school, it would be even less appropriate for younger audiences, and would be treated by the media as the game Bully was. So the second point I'm making is: games whose universe are rooted in fantasy can get away with more mature themes because it will have less impact on a younger minds who can distinguish between reality and fantasy.

tl;dr: read the stuff in bold.