Bioshock > Splicers > Aggressors: What Are They?
An aggressor, more commonly known as a splicer, is a citizen of Rapture genetically enhanced with the gene-altering substance Adam. Aggressors are all that is left of the citizenry, as war and murder have killed off almost everyone else.
The aggressors have over-indulged in Adam so much that they suffer extensive physical damage, especially to their skin, making them look molten or tumorous. Adam also destroyed their brains, resulting in madness and violent hostility.
The plasmids, that made the aggressors what they are, were also spiked with the same mind control plasmid used to condition your avatar. Pheremones are then used in order to control the aggressors' behavior as needed. This mind control makes the aggressors an important tool for Ryan (early levels), Cohen (Fort Frolic) and Fontaine (later levels).
When they are not being manipulated, aggressors wander the halls and rooms of the city. They bang on doors and gripe when the doors don't open. They bang on vendors and gripe when the vendors give them nothing — or when it is not a vendor at all but a turret or other device they are mindlessly working on. For Adam the aggressors scavange corpses and hunt for others of their kind and for Little Sisters. They need the Adam both carry in order to maintain their pathetic lives.
Yet, while their reason is destroyed, they are not bereft of humanity. They still have memories of what they used to be, as expressed by their babblings. And they usually wear masks, as Atlas mused, in order to hide the shame of what they have become.
When an aggressor sees you (or react to the Enrage plasmid you douse them with), they will kneel slightly, swing their arms back and scream like a banshee. Then they attack with all-out abandon. Aggressors usually attack singly, but sometimes gather in groups of up to four people, either to hunt you down or to attack a Big Daddy. (Mass gatherings of more then four people are discourage by Ryan.) If you use a Decoy plasmid, they will attack the dummy. If (like in the original Doom!) one aggressor hurts another, the injured party will attack the aggressor with the bad swing or aim.
All aggressors are physically strong and can take a lot of punishment, especially at the later levels. They can also dodge and weave when you aim your weapon at them, even if you are invisible from Natural Camouflage. Searching their dead bodies yields money, ammo, and other goodies. You get other goodies by taking pictures of aggressors (and other enemies) with a research camera.