How does Καινω become a ρημα VERB in the Greek lexicon? And yes the spelling of Cain in the LXX is Καιν. When did all this happen?

Καινω: to kill, slay, slaughter
(1st person, active voice, indicative mood)

Future tense: Κανω
Imperfect/2nd Aorist: εκανον

Και εγεντο ηνικα Καιν γεγονεν ως Καινω
and it came to pass when (Cain:proper noun, name,)became as (verb:to Cain.)