You should have a mental test ,though, to see if your mental capacity can withstand violence like that, but depending on your mental capacity you should be able or not be able to buy those games. But its totally different if your under the age of 10 and are playing those games because at that time kids have a huge amount of imagination and everything becomes killing things.
A moral choice can only apply when there is a dichotomy between a morally good choice and an immoral one. As morality is a standard of behavior that relates to the interactions between sentient beings, it's impossible for the purchase of a physical item, in itself, to be a moral choice. We only say something is 'immoral' if someone(else) is ultimately hurt by that choice. As nobody but oneself is affected by the decision to buy a video game, of any sort, it's impossible for it to be a 'moral' choice.
There's a big controversy now about whether or not video games promote violence - and maybe they do, under certain circumstances (I've never read anything to indicate the games themselves are directly linked to violence, though there seems to be evidence that they do desensitize kids to violence), but I don't think that it's immoral to buy an fps game. Also, people who would actually be buying games like that (as most are rated M) would be adults, who wouldn't have the same issues as kids with desensitization. Violent fantasies have always been part of the human psyche, and if shooting people in a game helps relieve stress (and helps prevent people from shooting people in real life), I think that's not a bad thing at all.
There is nothing moral about choosing one form of entertainment over another. Psychological? Perhaps. But moral? Certainly not! If morality was as black and white as this, then we would have no wars. We would see an aggressor and lock them up for crimes against morality. Instead, we have people who are each free to choose what is and is not okay in their world view. Shooting my friend in the head in a video game is a-ok in my world view! Taking that outside of the game and actually shooting them, however, would represent and underlying mental imbalance (such as a psychotic break). Never having played the video game might have prevented it. But, then turning on the news would present a whole other list of options for the altered or susceptible mind.
The decision to buy a shooter is a common sense choice, not a moral one. I get it our kids are growing up faster they know how to swear earlier blah blah blah but when the game is supposed to be for mature audiences, that means it's for adults. It's not morality that puts Halo in the hands of a bunch of 10 year olds, it's moronic parents.