The climax of the last paragraph leads to a diagnosis of the disease that poisoned quarrelsome Jewish communities. Faction fights were the logical outcome of unbridled passions; they campaign against man's self (1 Peter 2:11), and weaken his power of control.
James 4:2 is best rendered, You covet, and miss what you want then you murder. Aye, you are envious and cannot get your desire then you fight and wage war. It is hard to see how faction that would not stick at bloodshed could be found in a primitive Christian community; among Jews it is easily illustrated. These adulterous souls (James 4:4) have broken the marriage vow that unites God and His people; men cannot serve God and mammon, or give friendship at once to God and the world they are powers at war, and neutrality cannot exist.
James 4:5 is best taken thus: Or do you suppose that Scripture means nothing when it tells us He is yearning jealously over the spirit He made to dwell within us? The reference is perhaps to the general tenour of revelation, rather than to a single passage: there is no OT text verbally near to this. Nor is God's yearning a vague sentiment, it shows itself in His offering more grace the declaration is proved by Proverbs 3:34. Note how Peter takes up James's words, as often (1 Peter 5:5; 1 Peter 5:9). For the Christian the assurance is guaranteed by the resistance of Jesus to the devil. Sinners are to put away sin from hand and heart (cf. Isaiah 1:15 f.), and by penitence seek pardon. For an Oriental, fasting and lamentation were the spontaneous and natural expression of deep sorrow. Our Lord permits but never prescribes it, only insisting that it must be absolutely sincere and not for show (Matthew 6:16 ff.).