Warhammer 40,000: Space Marine.

tl;dr: Unbelievalbly violent, unbelievably fun. IYLTSOT,TITSOTYWL.

This is undoubtedly the most violent video game I have ever played. It is also one of the most fun.

You play a Space Marine in a "universe of unending war." The Warhammer universe is incessantly depressing: every planet the empire has occupied has been allocated to a single role, none of which are meant to support human beings, just the products of human beings: food for armies, military equipment for armies, foot soldiers for armies. You're the best of the best, a soft-spoken, English-accented space marine named Titus who calmly wades into battle and slaughters Orcs by the thousands.

And it is thousands. Titus wears highly effective powered armor, carries a handgun in his left hand and a sword, axe, chainsaw, or warhammer in his right. In one vast battle which lasted almost an hour, an Orc mage summoned wave after wave of orcs of all sizes and strengths, and I weilded my warhammer to slaughter them in unending, beautifully rendered three-d battle. At times of low hit points, I could summon "the fury of the emperor" or use a heavy, armored stomp to stun a particularly hefty Orc, if I had weakened him enough, and then issue an 'execute' command, which would result in an especially gory slow-motion blow of blood and violence, and my hit points would go back up.

It's your typical Warhammer 40K scenario: Orc invasion of a Mechanicus world after a source of unbelievable power. You wade through all the usual places: ruined cities, sewers, high-tech construction centers, military establishments, and so forth, battling non-stop waves of Orcs.

There's no real plot. The engine is a last-generation 3D engine, on par with Quake III, Halo II and the latest Unreal. It's more than adequate for this excessive machinima violence.

It is also fun. Undeniably, incomprehensibly fun. It's third-person melee combat done right, and there's nothing in it that will make you a psychopath. The orcs are unrelentingly evil, there's no moral ambiguity here, and there's no confusion with the "real world." It's great stuff. It makes no excuses: if you want to wade into the enemy, and this game give you that in droves.

If you like this sort of thing, this is the sort of thing you will like.