Even though Ymir is already dead before this story begins, he plays an important role in it – after all, the entire earth is made from blood, muscles, brains, and bones (and a few other bloody bits too). The story "The First Gods and Giants" tells how Ymir was the first conscious being to emerge from the fire and ice that whirled in Ginungagap, the in-between land. The beings that come after him, the Aesir gods (including Odin) decide that they can't begin their creation until they kill Ymir, who has always been wild and crazy, but has become totally uncontrollable in his old age.
They don't just kill him – they rip his body to bits him and make their new world from his body parts. It's like a gory kill sequence in the type of video game you mom absolutely does not want you to play. (God of War III comes to mind…)
Why read such a gross story? Good question. Symbolically speaking, the death of Ymir might represent the taming of the wild and uncontrollable forces in humans that must occur before creation can. It also represents the taming and re-shaping of the ugly and chaotic into something beautiful and orderly.
Does that symbolism make Ymir's children feel any better? Not a chance. They watch bitterly as their dad's body is torn to shreds. This murder begins the longstanding feud between the races of the giants and the Aesir gods, whose hatred for one another will continue forever.
Odin and his brothers are the third generation of Aesir gods. Their names come from the Proto-Norse Wodin, Wili, and We – words that mean knowledge, will (to do something), and spirit. In other words, Odin, Vili, and Vé might symbolize aspects of human consciousness which, working together, create life. The creative act that the brothers perform in this story involves transforming the parts of Ymir's body into the parts of the world, then separating night from day, and creating the races and animals.
Still, the three brothers' creation of order and beauty begins with an act of horrible violence: Odin, Vili, and Vé spill so much of Ymir's blood that it forms an ocean. Then they tear his body to bits.
Do you find it a bit odd that the creators in Norse mythology are also kind of homicidal maniacs? Maybe their mythology reflects the violence of Viking life, in which wealth and prosperity came from success on the battlefield.