The official review says that this game is "not for kids." I think that this is an overstatement. Yes, the game can be violent at moments, but it really isn't anything a parent should be extremely concerned about.
As far as killing goes, the vast majority of living things that end up being killed are aliens and robotic-themed AI (artificial intelligence). Some humans do die, but unlike some games, it isn't long and drawn out. There is one cut-scene, however, where the primary antagonist of the game uses a weapon called the composer to "compose" a ship full of humans. This means that they are disintegrated into binary code, and they can be seen stripped apart. As gruesome as that sounds, It isn't really gory. Skeletons and a layer of muscle are depicted, but they don't melt into a pool of blood, or something especially gut-wrenching.
Concerned about language? Although the game isn't technically rated for language, there IS some in the game. Curse words are few and far between, and nothing especially strong is said. But, as I said, there is some offensive language that you should watch out for, but all of it is in cut-scenes that can be skipped.
There is absolutely no sexual content of any kind in Halo 4. The AI Cortana is as close as it comes, with a slightly suggestive appearance. But as far as explicit content, there is absolutely none, and you need not worry about it.
Likewise, there is no drinking, drug abuse, or smoking in the game. Although a character can be seen smoking in previous Halo games, there is nothing in that regard in Halo 4.
There are some good role models in this game. Almost every "good" character is a fine example of bravery and sacrifice, and go out of their way to help their fellow soldiers.
As far as the overall message, it isn't very clear. The plot delves into a whole "man or machine" conflict with the Master Chief, which he isn't sure about in the end. There isn't a really negative ending, but it does end on a low note, as an entire city is "composed" (don't worry this time, it isn't up close and personal, there's just a camera shot of an empty city) and the antagonist, the Didact, somehow survived.
I hope this review helped you decide if Halo 4 is appropriate for your child, and was helpful in general.