The Metroid"You cannot kill what you did not create..."
The game presents no information on how the Metroid were created, or why they were created. I took it upon my hands-I'm taking many things in my hands aren't I- to create the ideas of how they were created.
The Metroid are created by a certain Chozo entity, the one of good evil, for a general good purpose, to make a new kind of new war weapon. Created from the souls of the Passed, the dead, they are nigh invulnerable.
The idea of them extracting the energy from its victims is an illusion, because energy cannot be created nor destroyed. They extract the soul from you, the very essence of living, and recreate you into a Metroid. They are highly intelligent, so high, they can make their escape attempts look like an accident.
It may seem that the Pirates are the ones trying to control the universe, which in reality, the Metroid are trying to. They use the Pirates technology against them, making their research look like its true and scientific, but its really an illusion. In time, they will turn and control all.
Why aren't they doing so now? Only one stands in their way, a Chozo. The Metroid had the entire Chozo race killed because they knew their weakness, the Hatchling. Now only one stands, Samus. When she is gone, all hope is lost.
The Mother Brain
When the Metroid were brought back to Zebes, they found the Mother Brain, the central intelligence entity. If they could control her, they would know everything. Two Metroid escaped from a lab, one created a diversion, while the other sacrificed itself. It was crushed in the machinery that brought the Mother her energy supply. When its blood was mixed in with hers, it then "possessed" her; all Metroid have a mental link with each other, so one is not superior to the next. Now they have a mental link with the largest information database in the universe.
The reason Samus has to kill the Mother is that she has become superior to the rest, due to its mental state. Therefore, if she dies, the rest are weakened highly to a state if everyone can work together mentally they will die. If they believe that fear does not exist within them, then the Metroid will die. The Metroid thrive on your soul and your mind, if you believe in what they can do, they will do it...