“We know that we have passed from death unto life, because we love the brethren. He that loveth not his brother abideth in death.”
1 John 3:14

He makes clear the reality of those who do not walk in love- they are in a realm of death and do not have eternal life because Jesus made clear that anyone who truly has eternal life has left that realm of death:
“Whosoever hateth his brother is a murderer: and ye know that no murderer hath eternal life abiding in him.” 1 John 3:15
So he calls them murderers, people who do not mind killing (the reputation of) others just because they can; he vocalizes the potential of such folks and he expects us to use the word to vet our hearts, desires and expressions!
“And this I pray, that your love may abound yet more and more in knowledge and in all judgment; That ye may approve things that are excellent; that ye may be sincere and without offence till the day of Christ;” Philippians 1:9-10
If love clouds your judgement he says you’d make wise choices, you’d live an unblameable and sincere life until the day of Jesus- meaning nobody can fault a man who has decided to walk in love towards others; the man who is determined to be patient, kind and believe the best when he meets others, even if he think they’re making a mistake as against opening them up to slander and ridicule because of some perceived differences.

Bible Reading Plan: Isahiah 42 and Leviticus 7