So in Serenity it’s revealed that the Alliance, in an attempt to pacify the population, released a gas (Pax) on the planet Miranda. The result is that 99.9% of the population became so passive they stopped doing anything and died where they stood. The rest became reavers, the boogie men of the series.
Now the idea that the Alliance would use gas to control their citizens makes sense. What doesn’t is the way they went about it. It’s really stupid to gas an entire planet, with millions of people, without doing any small scale tests. If they had tried it on even one person first the problem with the gas would have been discovered. In the film the effects of the gas comes as a complete surprise to the researchers that released it.
It would have made more sense if the Alliance had used the gas during the war as a method to kill resistance, while leaving infrastructure intact, but then being totally surprised by the creation of reavers as the odds of that happening was too low in trials.