"Save the World - Kill Yourself"
The Malthusians are a fanatical terrorist group and the main antagonists of the video-game, Hydrophobia - they share some similarities to the real-world "Human Extincion Movement" - however, they are far more radical than most real-world groups of this philosophy.
The game is set in the mid 21st century when the world has fallen into the chaos of the "Great Population Flood", and takes place aboard the Queen of the World, a city-sized luxury ocean vessel, built by a group of corporate giants known as the Five Founding Fathers; the group who, due to the Queen, have prospered while the rest of the world suffered.
At the beginning of the game, the craft is bombed by a group of fanatical terrorists known as the Malthusians—named after political economist Thomas Malthus who predicted that population growth would one day outpace agricultural production, returning society to a subsistent level of existence.
The Malthusians have a plan to murder the vast majority of humans on the planet, so that the survivors wouldn't suffer from the effects of the population explosion. Their slogans, including "Save the World - Kill Yourself", are written on the walls and displayed on computer screens all over the ship.
Kate Wilson, the protagonist, is a systems engineer who becomes a reluctant hero when the Malthusians attack and take over the Queen.