The overmind does not consume humanity.
The overlords (acting on orders from the overmind) interfere in human events in order to prevent us from destroying ourselves before reaching the point where humanity can transform itself into a being similar to the overmind.
The overmind can recognize that a race has the potential to become like the overmind. It uses the overlords (whose race does not have this potential) to help those proto-overminds to "grow up."
At the end of the book, humanity doesn't disappear or be destroyed by the overmind. Humanity transforms into an entity like the overmind.
The overlords force humans to settle down and be peaceful so that the last stages of the transformation will go smoothly. This also prevents things like a nuclear war destroying us before the transformation is complete.
Really, none of the figures in the book are evil in any way. The overmind wants to see those races who can transform into something greater actualy achieve it, and the overlords act as a sort of "racial midwife" to help each race achieve the conditions for it to transform.
The transformation is a natural thing, not forced by the overmind or the overlords. It happens when enough members of a race live in peace without fear of each other.
The overlords brought about the conditions, the latent human psychic abilities did the rest.
As I interpret it, the overmind managed the transformation by itself. I assume it was difficult and possibly traumatic, and that the overmind decided to help other races make the transition in a less traumatic way.
The overmind uses the race of the overlords to do the work of nurturing the races with the potential to transform into an overmind like existence. The overlord race doesn't have this potential, and maybe would have died out as an evolutionary dead end if the overmind hadn't put the overlords to work helping the proto-overminds.