A robot or other physically autonomous machine guided by a level of intelligence and a homicidal urge to Kill All Humans. Prone to ending sentences with exclamation points as well as Spock Speak. Killer Robots are so common because A.I. Is a Crapshoot.
Humanoid killer robots abound, such as the original R.U.R., or those in Doctor Who's unsubtly named serial The Robots of Death. However, killing machines come in all shapes and sizes; from robotic dreadnoughts in space to airborne robo-fighters, through small drones and down to microscopic or even nanoscopic killers.
Although the traditional versions of this trope were robots built in a Mad Scientist Laboratory who Turned Against Their Masters, today a more common plot involves the villain finding and turning the switch on a kid's Robot Buddy from "good" to "evil", forcing the cast to either destroy it or find a way to turn it back. Especially scary because this was a good friend who has become completely heartless and bent on death.
Mechanical Lifeforms are capable of veering into this, but generally tend to skimp on the Spock Speak in favor of something a bit more colorful.
Subtropes:
A.I. Is a Crapshoot
Attack Drone
Cyber Cyclops
Gray Goo
Hostile Animatronics
Humongous Mecha
Mecha-Mooks
Mechanical Monster
Morality Dial
Murderous Malfunctioning Machine
Robotic Psychopath
Robot Soldier
Robot War
Techno Dystopia
Terminator Impersonator
See "Second Law" My Ass! for a milder form of this behavior.
