release a toxin to create zombies but some would be immune and observe what happens to humanity, a psychological test of the human race after a while once the research into the human condition facing certain and inescapable death we would drop to the surface and save those who were worthy and intergrate them into our empire, of course psycho conditioning to treat the trauma would have to be employed, also the ones deemed unworthy would basically be the people who took advantage and killed and murdered and stole from other survivors they would need to take a pennance.
to clarify that they would be given armour and guns and would be dropped into the most savage and feral of the zombie hotspots, if they survive a year of constant fighting they are forgiven if they die their crimes are also forgiven.