A Nega-Drive, also known as Mind Negative Drive, was a unique aspect of programming in the Series 4000 mechanoids created by DivaDroid International, and an in-joke by a disgruntled mechanoid engineer.
The Boys from the Dwarf first noticed severe erratic and irritable behavior in Kryten when he followed them into the AR Suite during a game of Pride and Prejudice Land. Lister had only agreed to participate in an attempt to impress Kochanski, whom Kryten was already jealous over. Kryten stealth killed the Bennett sisters, and then blew up the gazebo with a T-72 Tank, all because they postponed Kryten's lobster dinner. At said dinner, Kryten's head literally explodes when Lister asks for ketchup with his lobster. He then blows up all his other spare heads when he puts them on.
Later kidnapped by the Simulant Trader aboard the SS Centauri, Kryten meets another Series 4000 mechanoid Able, an outrozone addict, and learns the terrible secret of his design. It is revealed that the entire 4000 series of mechanoids was a spiteful joke by their creator, Professor Mamet. They were designed to be a parody of a fiancée who jilted her, and as such were made pompous, ridiculous looking, and overbearing. Further, all their negative emotions are stored on a "nega-drive" and when it becomes full they literally blow up. It seems likely that the only reason DivaDroid developed the model was due to the reception of the Series 3000.
When Starbug came under attack by the laser cannons of the SS Centauri, Able took an escape pod and fired a green energy field at the Centauri. This energy field contained all the bad feeling built up in the nega-drive, which in turn infected the Simulant Trader.
In a fit of depression and in a blaze of self-hatred, the Simulant Trader committed suicide by exploding the Centauri. However the explosion also killed Able. Kryten recovered the body, finally calling him his brother. ("Beyond a Joke", Series VII)