Scenario: an alien race has traveled across the gulf of time and space, let's say from the Alpha Centauri system, fleeing their own dying world and looking for a suitable replacement. They find out about the earth as they enter the solar system.
Now, obviously if these guys are capable of interstellar travel, their technology is far beyond what humans have. But there are two limitations:
they have the tech, but not the numbers. Keep in mind that these guys are essentially refugees, tired and exhausted from their long journey. Maybe there are a few thousand, maybe tens of thousands, but there are way, WAY more humans.
they need the earth to be in more or less intact condition for them to recolonize it, so dropping an asteroid onto it or nuclear weaponry is not ideal as they would have to wait a long time for the planet to recover - they'd rather start the colonization process now. Let's say that they used up all their resources getting to this point and they really can't wait around for the years it would take for the earth to recalibrate.
What can the aliens do to cut down on the human population? Perhaps some kind of bio weapon that kills most people? This way the aliens can then put boots on the ground and just finish off the rest?