It is true that the word being questioned in 1 John 3:12 has been variously rendered, 'murdered', 'slew', 'slaughtered', 'killed'. It is also true that differentiation is made in the Old Testament between murder and sanctioned warfare, where the carnage of killing enemies deliberately is not put alongside premeditated murder. There is a matter of legality here.
Of course, there was no 'war' going on between Cain and his brother Abel. There was no sanctioning of any action resulting from the jealous brother. Indeed, God warned Cain (knowing the hatred in his heart), giving Cain time to turn away from the outcome of harbouring such hatred.
By today's Western standards Cain might be judged to be a man-slaughterer, instead of a murderer, for the 'only' witness to the crime was God. Yet predetermined actions took place; not spur-of-the-moment loss of control, nor due to antagonism on the victim's part. Cain chose the location and the time, with no witnesses present to see him carry out what he planned to do. Yet 'murder' is a judicial term to be applied after a legal judgment about a case has been made. Is that why it would be wrong for any translation to pronounce 'murder' when there were no witnesses, and God never said 'murder'?
Nobody should be surprised at the NIV (and other translations) saying in 1 John 3:12 that Cain murdered his younger brother. However, the Greek word is σφάζω sphazo, which only occurs here and in Revelation 5:6, 9, 12; 6:4, 9; 13:3, 8 & 18:24. This word means 'kill, slay, wound'. The word sphage means 'slaughter'.
This is where the NIV Bible Commentary (1986 edition) is helpful. Up until at least 1987 the NIV said 'murder'. The relevant part of the explanation states:
"In Cain is seen the horrifying nature of hatred: he murdered, and
that his brother. And why did he murder him? ...The apostle has no
time for moral sluggishness which refuses to see ultimate principles;
and brushing aside all possible secondary and contributory causes he
declares that the sin was Cain's. Cain could not bear the contrast of
his brother's righteous actions with his own actions which were evil,
and the first murder became a monument to self-love." NIV Bible
Commentary, p. 1580, article by R.W. Orr (1986 ed)
This certainly has implications for what Genesis 4 states. But, 'interpretation'? Here is the NIV Commentary again, on saying 'murder':
"As was said on 3:7, the first effects of sin were seen in the family,
and it is entirely consistent with this that the first murder is
fratricide... No suggestion of previous tension between the brothers
is mentioned... We should not import any redemptive meaning into the
brothers' sacrifices, cf. 8:20; they were the recognition of Yahweh's
lordship. Both gave of what they had... The reason for the rejection
is suggested in vs. 6, 'If you do what is right'. The probability is
that he resented having to accept God's lordship (cf. v.13).
The Hebrew of v.7 is difficult, but we need not doubt that the general
sense has been given by NIV when it sees sin personified as a ravenous
beast. Speiser's rendering, 'sin is a demon at the door', is worth
mentioning.
8. Cain said to Abel 'Let's go out to the field': i.e. the open country. The addition to the Hebrew text is necessitated by Hebrew
linguistic usage, and is in all the versions. It suggests that the
murder of Abel was premeditated... Abel's blood was crying to God for
vengeance (v.10) and therefore it was not the responsibility of others
to take vengeance." (Ibid. pp.118-9)
This is why the NIV gives 'murder' both in Genesis 4 and 1 John 3. And the Apostle John, writing his first epistle, did not need to 'interpret' anything with regard to stating what the crime of Abel was. He did not have 21st century ideas about such matters, as the rest of his surrounding verses show. Indeed, John quoted the words of Jesus, heard with his own ears,
"Anyone who hates his brother is a murderer" [Matthew 5:21-22], and you know that no murderer has eternal life in him. [Galatians 5:20-21 & Revelation 21:8] 1 John 3:15 NIV