Cyberpsychosis is a mental illness caused by adding too much technology to your body. The Cyberpsycho loses their sense of identity as a person. They come to view regular people and other living things as weak and inferior. With their enhanced physical abilities and complete disregard for life, Cyberpsychos are extremely dangerous to anyone that crosses their path.
Cyberpsychosis can affect anyone eventually, but the less empathetic a person is, the more vulnerable they are to this mental disease. Someone who is warm and friendly is much more resistant than a psychopath, who already doesn't identify with people to begin with. But if you pack enough technology onto them, anyone can succumb eventually. There is no real cure (though there is treatment...see below). The effects are cumulative. A single cyber hand probably won't push you over the edge. Replace all four limbs and both eyes though, and you'll probably start down the slippery slope to madness.
The symptoms are gradual...at first they may simply become more distant to friends and family. They start to identify more with machines than people, and stop doing things that used to give them pleasure (eating, sleeping, socializing, sex, ect). Eventually human interactions become irritating, and then irritation morphs into contempt, and finally violence. Not all cyberpsychos are physically violent. Some have fragmented personalities, some are kleptomaniacs, others are compulsive liars. But violence towards living things, especially people, is most common.
Cyberpsychosis has become a major problem in the technology-saturated world of Cyberpunk. It is actually quite common in many boostergangs, though fairly rare in polite society. Police departments have heavily armed and armored units, called Psycho Squads, who specialize in subduing or killing cyberpsychos. Psycho squads are the SWAT of the cyberpunk world. They are well armored and armed to the teeth.
Incidentally...the government (and corporations) are not going to wait for you to go over the edge. It is quite common for governments or corporations (who are often the defacto government) to track people who are known to install lots of cyberware. This is another reason ripperdocs exist...not everyone wants to be on a government watch list for cyberpsychosis.
Registered Cyberpsychos and Therapy Edit
People who are near the edge are politely asked (that is, required) to register. A registered Cyberpsycho is on something like parole. They must attend periodic therapy sessions to keep them human. They have implants to monitor their activity. So long as they don't do anything nutty, they can go on with their lives as normal.
A Cybersphyco who goes over the edge is often killed outright. Cyberpsychos usually have enough implanted hardware to cause significant damage, and are a threat to anyone they come across. Most police departments will not go out of their way to risk people's lives trying to take the psycho alive.
But if the Psycho can be subdued, they will be strapped down and all of their cybernetics deactivated. They are forcibly jacked into a custom braindance, where a Cyberpsychologist (sometimes called a Psychoshrink) attempts to piece their personality back together.
Over weeks or months the Cyberpsycho gradually regains their humanity. They may even eventually become human enough to install cybernetics again (assuming they are not in prison for crimes committed on their rampage). But they are never really the same again. Mental damage from cyberpsychosis will change aspects of their personality, and follow them their entire lives.
FISK, C. Cyberpunk, Version 2.0, 1st ed. Berkeley CA: R.Talsorian Games, 1990