You have to understand that they have enslaved many many other planets just as they have done this one. Same tactics over and over. Many of the other planets inhabitants did not wake up and are fully enslaved.
The would have simply killed us all off by now it it were not for the enormous amounts of negative energy humanity was feeding the entities at the top.
They are addicted to it. It is their life source. Killing us off would mean they would lose their food supply.