As a highly advanced alien race, they have discovered that critical applications of their hybrid organic/AI technologies work exponentially better with human DNA than with their own planet's genetic materials. They need a lot of human DNA since their empire is vast. Thus, they have no intention of killing us. They want us to reproduce.
So why wouldn't they genetically engineer the necessary DNA? Lets just say that nature makes exactness impossible and that all attempts to produce synthetic human DNA quickly become unstable.
EDIT To be clear regarding genetic engineering. There is only one type of life on planet earth and all of our g.e. technologies use these genetic materials. We do no synthesize alien genetic materials.