This is a wonderful game. I love this site, I use it myself to look up a few specifics concerning ratings - but violence and gore is one you really have to watch out for, here. As they tend to overexaggerate a great deal with that, specifically. Half-Life 2 follows the path of a Gordon Freeman, a former scientist who's found himself caught up in playing a key role in what may very well be the salvation of human kind. The world as we know it has been taken over by an alien controlled "Combine" government, and it is implied that they can be brutal, cruel, and unjust towards the human population. However - this is NOT depicted in a ridiculous, in-your-face, "for the sake of it" manner. It is depicted in a very emotional and story-relevant manner. You will in your travels through the game, run across some corpses. Some apparently burned, some just dead comrades fighting for the same thing you are.
But, while realistically depicted, it is not depicted for the sake of violence - but for the sake of a very moving and solid storyline. It is immersive, and when you see it - you don't think "OH MY GOD, THIS IS VIOLENT." You think "Poor guy. The combine is ruthless, and needs to be stopped."
90% of the actual battles in the game do not consist of hardly any blood or gore. There are some zombies, and they obviously look like zombies partially covered in blood and a few of them can be chopped in half - but again, it is not inappropriately done for the sake of violence, only realism. Most of the time, you're fighting combine soldiers, and you're lucky to see a blood splatter. You just shoot, they fall (or fly back against a wall), and that's that. Not bloody, not gory.
It is a realistic depiction of an oppressive and potentially violent system, but it is a very involving and mature presentation of it. Any violence you encounter will be relevant to the storyline and to the world you're involved in, and I would argue that games like this are WELL within the scope of what anyone over the age of 13 should be exposed to. They'll see a hell of a lot worse on TV, or your average movie cover. This isn't your average "shooting game", this isn't close to Doom 3. This is something entirely different. Something I wouldn't want an 8 year old to see, but something your 14 year old son or daughter should be able to get involved in if they really want to.