And not be like Cain does not have a verb in the Greek. The phrase expresses a negative obligation, contrasting to the preceding positive one. In English and several other languages, this is best brought out by rendering the phrase as a new sentence, adding a verb form; for example, “we must not be like Cain” (Good News Translation), ‘we should not do the same as Cain did,’ ‘do not do like Cain did.’ Such a verb should parallel the one used in verse 11b. This reference to Cain is the only explicit reference to an Old Testament person in John’s Letters.

Who was (literally “he was”) of the evil one and murdered his brother: the Greek does not have the relative pronoun. This use of a separate sentence is to express better the unexpected horror of the case. This is brought out, for example, by ‘that devil’s-child which murdered his own brother’ (Bijbel in Gewone Taal).

For “to be of the evil one,” see comments on “to be of the devil” in 1 John 3.8. For the evil one see comments on 2.13.

The Greek verb rendered murdered here is a rather strong one. It was used of butchering or slaughtering a sacrificial animal, then of killing a human being by knife or sword, or more generically, of any form of murdering a man. Its use here is to express violent passion. In languages where terms for killing must specify the instrument, one should not use a term associated with knife or sword, but one referring to killing by hand, stone, or club.

His brother: the reference of the pronoun his is ambiguous, on the surface at least. The translation should make clear that the reference is to Cain, the subject of the sentence, not to the evil one. One version excluded this ambiguity, for example, by adding the proper name, ‘Abel, his-younger-brother.’

In contrast to its other occurrences the noun is used here in the sense of “bodily brother,” but verse 15 shows that the application is wider, namely, to man’s relationship with his brother in the metaphorical sense of the word. In some languages it has to be made explicit that the brother was younger than Cain, or that they had the same parents.

Why did he murder him, or ‘what was the reason that he murdered him,’ ‘what caused/led him to do so.’

His own deeds were evil and his brother’s righteous, or ‘all he did was evil and all his brother did was righteous,’ which is agrees with the fact that the persons concerned were “of the evil one” and “of God,” respectively. Thus Cain is the typical example of the children of the devil, who act according to the will of the devil, just as Abel is the typical example of the children of God, who act according to the will of God (compare Matt 23.35; Heb 11.4). And is preferably “but,” since it marks the contrast between the two parts of the clause.

For righteous see comments on “just” in 1.9. The word is used here with reference to deeds, not to persons, as is the case in the other occurrences in this Letter. This may make necessary some adjustments in the renderings commonly used.

Quoted with permission from Haas, C., de Jonge, M. and Swellengrebel, J.L. A Handbook on The First Letter of John. (UBS Handbook Series). New York: UBS, 1972. For this and other handbooks for translators see here .