It makes no sense that the Alliance would create the Reavers. If anything it makes sense that the Browncoats created them during the war to attack Alliance ships.
Reavers were originally humans from the planet Miranda and the Alliance created them using Pax (G-23 Paxilon Hydrochlorate). The Alliance was testing the chemical to see if they could placate the population. 99.9% of the population became so "peaceful" that they stopped eating and simply laid down and died right where they were. The other .1% turned into Reavers, because they had an extreme, adverse reaction to the chemical.
The better question to ask is why the Alliance covered up their failed experiment with Pax.