Is the Mechanical Hound from Fahrenheit 451 a possibility in the real world?
The Mechanical Hound is a robot built to hunt and kill dissidents in the world of Fahrenheit 451. It can sense a person's unique biology, described as "amino acids" and other chemicals, and then kill them with a poison that works on their body chemistry. While variations of this are possible today, there is no autonomous spider-legged robot that can run around in a city and kill people on an individual level.
However, there is a parallel today with the rise and use of drone airplanes that can spy on people and then kill with missiles; these are used throughout the world for military purposes. These drones are still not autonomous; for precision spying and strikes, they must be radio-operated from the ground. However, they have become more and more prevalent because they do not put human operators in danger; there is a recent push-back against the use of drones over U.S. land to search for drugs, weapons, and other illegal activities. The use of drones to kill selected targets may be seen as analogous to the Mechanical Hound, although as of yet they not as common or as versatile.