R rated games is less of an issue than you think. Before R rated games were a thing there were less than a dozen games that were refused classification and of those most had some minor tweak to get it past the censors (e.g. changing the name of one item in Fallout 3/NV so it doesn't reference morphine, stopping you from beating hookers to death after hiring them in GTA3 etc)
The main reason the R rating was pushed by industry groups was to stop little timmy buying it, not to allow for more graphic content.
As for the studies, well they are actually inconclusive. For every well designed study finding no correlation between violent games and aggressive behaviour there is another that does find a link. Playing a violent game may not make you a killer, but it may exacerbate underlying issues. This isn't unique to games however, it applies to all forms of media consumption. Why do you think they put helpline numbers on every news article about suicide? Because reading the news article may tip a severely depressed person over the edge.