Why Ymir Was Killed in Norse Mythology?
The first being in the cosmos in Norse mythology was Ymir the giant making him among the most primeval beings in Norse cosmos.
When Odin and his brothers came to Norse world, they decided to kill Ymir and to use his body shaping the world. This story has always been in debates for scholars. So what is the point of killing the first giant? Three major points have been widely accepted: sacrifice symbol, natural selective process, and circle of fate.
Symbol of Sacrifice
Why sacrifice? If we pay attention to every Norse story, we find out that in Norse mythology, nothing was free, if you want something, you have to pay for it. Odin exchanged an eye to get wisdom, Freyr gave his powerful sword to win the heart of a giantess, or Tyr lost an arm to earn peace for Asgard. A thing for a thing, nothing would be gained if one refused to make a sacrifice.
So the first sacrifice in Norse cosmos was the death of Ymir. When Ymir died, what did the Norse cosmos get back? Many, indeed.
The body of Ymir the giant was used to fashion the world: his skull became the sky, flesh into the earth, blood into the ocean, hair into the forest, teeth for mountains, etc. There came the Norse world which was probably Midgard the Earth now.
The worms from Ymir's corpse also evolved into the dwarves who later became the most talented craftsmen in the cosmos.
The corpse of Ymir in Norse mythology
When Odin and his brothers walked along with the world that they made out of Ymir's corpse, they found two tree logs that they turned into humanity - Ask and Embla.
In some specific times, we have to make a sacrifice in order to gain something better. That's the first lesson of the story. The death of Ymir is nothing horrible. It is a sacrifice that must be made in order to make the wheel of life run.
Natural Selection Process
What is the meaning of a natural selection process? When new things come, old things have to move out leaving the show for the new ones.
When the time of Ymir finally came to an end, he had to bid a farewell to all the audience leaving the stage for the upcoming generation. If one keeps holding on to what is no longer up-to-date, they are embarking on a journey to travel back time living with the old things, not to say antiquated things.
So the death of Ymir is an example of this. He was old and the young generations were born. And even Ymir was the primeval giant, he could not escape the truth that he must die. Before the new things can be born the old things must perish. That's the rule of nature.
Circle of Fate
Regarding the circle of fate, it relates much to the gods. If you have done enough research on Norse mythology, you know that the end of Norse Pantheon was Ragnarok. I do believe that Ragnarok was caused by the gods themselves. The first reason for Ragnarok was the death of Ymir.
Gods in Norse myth are not idolized so much. They are the models of humanity, in fact. That's why we can feel their greed, conspiracy, and even their evil. They created their ending at the beginning.
Odin and other gods killed Ymir and the offspring of Ymir would slay Odin and gods. The death of Ymir from the start heralded the end of the Norse Pantheon at the end. That's the circle of fate and once fate knocked at the gate of Asgard, no one could escape.