When you have only a very small force available, it is difficult to control an entire world of increasingly evolved and educated humans, especially when there are those of your own kind on the surface working against you, and you cannot trust even your own allies.
To keep the human population under their control, to instil fear of the gods, to punish those opposing them, or to simply prevent a problem, the Anu began to resort to very drastic methods.
When a group of humans were causing trouble, they could expect fire to rain down from the skies as their Gods bombarded them with advanced weaponry. Death and violence, especially at children, created a lot of fear and anxiety and were used on purpose to gain more power.