Sacrifices, lots of sacrifices, i imagine they believe the more they sacrifice the more power/rewards they get.
Also they eat them and use the parts.( andrenalchrome and parts for their rituals/witchcraft BS)
Also after they have been used and abused, they need to get rid of many of them, and keep ones they really like, or who can work for them, make them money, or make good obedient slaves.
Most of them and all their family and friends are pedos, so they need an endless supply of babies and children.