"... The Hebrew of the verse is cryptic. It constitutes a rebuke to Cain from Yahweh God and probably means something like: 'Is it not: if you do good there is a lifting up (i.e. of the face), but if you do not do good, at the door sin is crouching; its desire is towards you, but you must rule over it.' Sin is personified as a wild beast crouching at the tent door, who is desirous over against you, but you must prevail over him. LXX Genesis did not understand it this way at all. Since the general context is that of offering sacrifices LXX Genesis puts a cultic interpretation on the first part of the verse. By taking it in the sense of raising a sacrifice and by rendering it adverbially the translator comes out with "Is it not: if you should sacrifice correctly." This contrasts with 'but you should not correctly divide,' i.e. divide or cut up the sacrifice. LXX Genesis considered Cain's sacrifice unacceptable to God, because he had not performed the sacrificial ritual correctly. Could the translator have read [Leviticus 1:12]? The line as a whole then may be rendered: 'Is it not so (that) if you should sacrifice correctly but divided (it) incorrectly you have sinned?' ..."

Wevers, John William Notes on the Greek Text of Genesis (pp. 54-55) Scholars Press, 1993