The Sliders land on a world where the entire population has been sent to live on another planet, causing all sorts of over-population related problems. The empty world has one remaining resident; Quinn's double.
This parallel earth Quinn caused the world wide slide, and has a habit of being impatient, and a wee bit cold hearted when his experiments hurt other people. Naturally, the Sliders visit the overpopulated world and figure out a way to fill the empty world.
The most interesting aspect of this episode is the relationship between the two Quinn's. The empty world Quinn can build a world wide sliding machine, but struggles to appreciate the impact that his experiments can have on other people.
Quinn tries to show his, empty world, double the importance of practicing science with a conscious. The role of ethics in science is an important issue to examine, and this episode does a pretty good job of exploring some of the basic challenges that come when scientists don't have a good ethical compass.
One local impact of the overpopulation, is that a former greeting cards salesman has set himself up as a powerful warlord. It's an interesting idea to see how different people have responded to the shortages, violence and general chaos caused by overpopulation.
Granted, the episode avoids dealing with real world ways ways to combat overpopulation, such as family planning, and returning people (the entire planet) back to their home world has got to raise all sorts of complicated legal and ethical problems.
Would certain people like the greetings card salesman be changed with war crimes? Would people be entitled to whatever land and money they had when they were put on the parallel earth?
Lots of questions are sidelined in favor of the the empty world Quinn learning to pursue science in a more compassionate and ethical manner.
"World Killer" works best if you focus on the differences between the two Quinn's, and how empty world Quinn gradually learns how to follow some sort of moral or ethical code. It is less effective as an examination of overpopulation or the complexities that would arise in returning an entire global population to their home world.