History
Cain was a drug dealer who sold the highly addictive designer drug Nuke. When Robocop raided their plant he was killed. OCP was in the process of trying to replace Robocop with a more complicit model. Other policemen proved unstable after being made a fusion of man and machine.
OCP chose Cain, counting on his addiction to Nuke to control him. The machine he was placed in ran on large quantities of Nuke. His criminal nature was unchanged and he went on a rampage. Robocop would destroy Cain after his drug fueled killing spree.