The two words are: Cain and lair.
Cain is a noun proper, the name of the biblical, fraternal killer of Abel. They were both the first sons of Adam and Eve and Cain killed Abel out of jealousy, becoming the first murderer of humanity according to the Bible (Book of Genesis, First Testament). By comparing the monster to Cain, the author is magnifying the gravity of its killings and infusing them with a sense of epic transgression.
Lair is a noun that means a secret, hidden place where a wild and ferocious animal sleeps. Now, because Grendel, the monster is likened to Cain, who committed the first fratricide, it is not only depicted as wild but as evil. Furthermore, Cain is usually likened to a demon or to Satan himself, and its lair here is an allegory of Hell.
"moor-stalker" is a compound word that tells the reader about the beastly connotation assigned to Grendel. Grendel lived in the marshes and followed and watched his brother ,probably, till he killed him. Grendel belonged to Cain's kinship and had also killed his brother ; Cain had killed Abel. Grendel 's wild and frightening spirit is also highlighted when the writer tells he lived with monsters in their lair /cave.
"Kin-slaughter" is another word used by the writer. This word is used to refer to Cain but it also refers to Grendel. Both Cain and Grendel had slaughtered their own brothers. To slaughter is much stronger than to kill.
