It's disconnected from life to a limited extent. I can't shoot an alien with a plasma rifle but I can sure shoot a human with a ballistic one and in the same manner. The observation of violence is still there. The motor reproduction of aiming and pulling the trigger is still there. Motivation is still there as well though it can vary: from disliking the alien/human, perceiving them as your enemy, to perceiving some reward in the end. It is these steps that define imitation, not the superficial context of what kind of weapon you are using or the species of the person you are shooting.
ESRB Fantasy violence: violent actions of a fantasy nature, involving human or non-human characters in situations easily distinguishable from real life.
I don't think this rating could be applied to Halo. You shoot a living being with a gun and blood comes out; that's really more Blood violence/blood and gore. A better example of fantasy violence would be the Legend of Zelda: The Wind Waker. When enemies are hit with a sword, they reel back as if punched, and light forms between the enemy and sword as a hit marker.
Al Man: Okay. Not all people will react to something the same way.
Christian guy who's name I forget: Not published. Just an essay I turned in for a grade. I hope that doesn't make anybody think I'm a know it all college student or anything. Some sources that people might want to check out is reference book Playing Video Games: Motives, Responses, Consequences by Vorderer and Bryant.
The correlative study I used was The effects of violent video game habits on adolescent hostility,
aggressive behaviors, and school performance by Douglas A. Gentile,*, Paul J. Lynch, Jennifer Ruh Linder, David A. Walsh.
Social cognitive theory by Albert Bandura is also worth a look, because I believe it best explains the relationship between aggression and video games.
I can upload my paper to the internet and leave a link if you guys are really interested.