Paul was writing to believers who were arguing with each other.  The issue was whether or not you could be a true Christian if you did not also fulfil all the Jewish laws.  The legalists were so insistent that they were right that they behaved like wild animals, biting and tearing at each other with sharp claws.  

Paul then drew them back to the ‘Royal Law’ they were supposedly defending (James 2:8).  This comes from Jesus who had taken the daily Jewish prayer, the Shema, affirming the uniqueness of God and His right to be worshipped (Deuteronomy 6:4) and had added ‘and love your neighbour as yourself’ (Luke 10:27).
 
Paul inserts a necessary negative from Leviticus 19:18 which says, “Do not seek revenge or bear a grudge against anyone among your people, but love your neighbour as yourself. I am the Lord.”  So, Paul protests, how can God’s ‘law-lovers’ hate people so much!  It is an absurd contradiction.  The end of that process is destruction, not building up the church.
 
The ‘law of love’ is a good test for devotion to God.  Violent disagreement is as wrong in the church as it is in the home.  Hatred should not find a home among God’s people.  That does not mean that we should agree with people who teach and do what is wrong, or trust those who do not hold to the truth; but we should still love them.  Jesus did that: so should we.