My 2 cents
I think it kind of depends on the game.
In a shooter like the COD series, you don't have a choice really about the enemy. You're trying to complete an objective and survive. Games like these tap into the Hero fantasy that a lot of gamers have. Who doesn't want to be the hero and save the day. In these games, there really is no reason behind the violence or choice behind it
Then there are the more open sand-box RPGish games for instance Deus Ex or the GTA type games. Here, the game has the flexibility to allow the players choice and values come into play. For instance, *Possible Spoiler for those who haven't played the original Deus Ex*
There is a misson that you have to raid an airfield because "Terrorists" are off loading stolen vaccines. The first time playing it, I'll be going in there and wiping out all the opponents using my augmented abilities. At the end of the mission, it's revealed that these "terrorists" are actually ordinary citizens trying to get access to a vaccine to save their families and it's actually high level officials in the governement that's releasing the plague and using it as a means to gain control. Your character ends up joining the cause for the rest of the game.
Now knowing the full story, when you replay the game, it does cause some thought how to do that airfield mission again. Do you go gun blazing again? Non-lethal? The choice is yours. Interestingly enough, the deus games can be completed without killing anyone (except for some bosses where it's unavoidable)
In GTA type games, usually you'll have to kill a lot of people in your rise to imfamy. However, most players will also have no qualms about running down people, killing hookers and taking their money, and causing utter destruction. In this case, the game doesn't force you to do this.
I think that a sign of a really well developed game is one that makes you think about what and why you are doing what you do. This is why open ended games are generally difficult to play from an evil side (people are inherently good), why there was such a reaction when EA's Medal of Honor game got flack for using the term "Taliban" as a bad guy in multiplayer, and even the airport scene in MW2 and whether you choose to take part in the airport massacare.
My point really is that video games, much like any media product, is an escape. You can do what you always fantasized about in a safe environment and without repurcussions in the real world. It's the really well designed games that will cause you to think about your actions and that goes beyond just an escape.
Hope what I wrote made sense
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