I'm developing a society that uses methods of mind control and scientific enhancements on their military force and does extensive experimentation on test subjects without their consent. These include mind control, truth serums, torture experiments and controversial medical and biological tests.
The society's work force live in a walled city divided into districts and have done so for several generations and none of the current population remember what it was like to be free or live outside of the city. As the military also takes the role of the police force their primary job is to enforce the government's requirements on the population and to punish offenders. Many offenders find themselves undergoing extensive mind control and then are placed into military service.
Naturally the government scientists are constantly performing experiments (they have no ethical issues with experimenting on humans) and of course sometimes these experiments fail. These experiments are performed on "guinea pigs" before they are used on the military force (The guinea pigs are normally prisoners, etc). Sometimes the victim dies and sometimes they are severely mutated.
To add dramatic elements to my story I want these mutants to be released outside the city's walls instead of being destroyed and for them to have the appearance of black zombie-like creatures (not necessarily have all the characteristics of a zombie) that are extremely mobile.
Since this society is fairly advanced (50 to 100 years ahead of us) it could be that this particular strand of mutants could be from past experiments before the government got to its current state of knowledge.
My question is this:
What experiments on humans could possibly fail in such a way that could produce mutants with the appearance of zombie like creature that are still mobile?