As you work your way through the tragic story of amnesiac scientist-turned-mental patient Dr. Daniel Lamb, you'll lurk through seedy, mostly linear environments that include a decrepit porn theater, a burnt-out (and possibly haunted) house, an underground sex club that's actually a front for something far more sinister and - perhaps most surprisingly - a relatively peaceful suburban neighborhood patrolled by assloads of cops. Your survival depends on sticking to shadowy patches that keep you hidden from the maniacal hunters hired to murder you. In order to make the way safe, you'll frequently have to murder them back, and the best way to do this is to hide somewhere dark, make some noise to lure them over (either by tapping on the wall or throwing an object), sneak up behind them and execute them with whatever makeshift weapons you can find.
There's an excellent variety of these, ranging from pens and chunks of broken glass to axes, crowbars and M-16s. You'll also be able to use environmental hazards in your executions, such as open manholes, industrial machinery and puddles of gasoline you poured yourself; just lure a hunter next to one, attack when the targeting cursor turns red and you'll have a disfigured corpse to contend with instead of a live hunter.
The longer you can sneak behind your intended victim, the more brutal the execution; a quick kill might entail just a few stabs, while a more elaborate one involves you straddling an enemy while jamming a syringe into his eye socket. Just be careful not to wait so long that they turn around and see you while you charge up your brutality, or you'll have to clumsily fight them and whatever other hunters are nearby.
When the game begins, this might seem a little too simple, and you'll quickly fall into a pattern of hiding in the dark, banging on the wall and ambushing the first hunter dumb enough to turn his back on the big patch of shadow he knows you're hiding in. (Hunters almost never enter shadowy areas, although they will occasionally peer into them - at which point a quick minigame will determine whether or not they see you.) Things get a lot more complicated when hunters start roaming in packs, however; making noise will frequently lure them all over at once, and killing one without being seen by the others isn't easy. You'll also be frustrated by nasty surprises like automatic security lights (triggered by any movement faster than a slow crawl) and hard-to-see gravel surfaces that can give away your position in a second.