1 John 3:12

Cain’s murder of Abel evidenced control by Satan rather than by God. Cain was jealous because of Abel’s greater righteousness, and this motivated him to kill his brother (Genesis 4:3-8; cf. John 8:40; John 8:42; John 8:44). Often our pride tempts us to dislike those who are more righteous than we are because they make us feel guilty by comparison. This is the only Old Testament reference in John’s epistles and the only proper name, except for names of God, in 1 John. Love and hatred are typical forms of righteousness and sin respectively. [Note: Dodd, p. 82.]

Saying that Cain was unsaved and so an unsaved person must be in view here will not work. The Bible does not say that Cain was unsaved. Furthermore, Christians have committed murder, as Cain did. Peter warned his readers, "Let none of you suffer as a murderer . . ." (1 Peter 4:15). Christians are capable of any and every type of gross sin. It should be obvious that a true believer can hate his brother Christian.