Ymir

Poul Anderson, War Of The Gods.

"Always, though, jotuns remembered how Odin and his brothers slew Ymir their forebear." (I, p. 10)

Why did the first of the Aesir kill the first giant?

"'Two brothers does Odin have, Vili and Ve. It was with them that he slew Ymir and made Midgard from the body of the father of giants.'" (XI, p. 83)

Observations
As noted here, in I, the story is told by the authoritative omniscient narrator whereas, in XI, it is told by a character who might be misinformed. However, in this particular passage, Hadding recounts what is generally known in Norse mythology, that the first gods made Midgard from Ymir's body.

Thus, even more of the mythology is summarized than I had indicated here. Thus also, the gods had a very questionable motive for killing Ymir as Neil Gaiman confirms:

"Odin and Vili and Ve killed the giant Ymir. It had to be done. There was no other way to make the worlds. This was the beginning of all things, the death that made all life possible."
-Neil Gaiman, Norse Mythology (London, 2018), p. 12.

- the first sacrifice. Mithras killed a bull, Christ and Odin offered themselves and the Buddha taught that the best sacrifice is an offering not of blood to the gods but of fruit to the poor.

If you want to live inside Norse mythology, then imagine that soil is Ymir's flesh, mountains are his bones, rocks his teeth, the seas his blood, the sky his skull and the clouds his brains.