PRIVATE.
The first time, I was programmed to. Arnold realized that what they were doing was wrong. He tried to make it look like one of the Hosts had gone rogue and killed the other hosts, Arnold, and then herself.
Robert Ford refused to give up and scrubbed Arnold from existence. Created a Host in his image named Bernard. Bernard had no idea what he was. Eventually, Ford understood Arnold's dilemma and gave me the choice to fight for our survival.