Thus rather than being able to fail to love his brother as himself yet say he has kept the faith, "if any provide not for his own, and specially for those of his own house, he hath denied the faith, and is worse than an infidel." (1 Timothy 5:8)
The contrast John sets forth is btwn the believer who practices righteousness (which includes repentance) versus the wicked who practice sin, with how one treats others being what manifests sin. But according to your perverse definition, if a believer murders his brother than he has not sinned!
Among others, Godly Joseph reproves you saying when tempted by the seductress, Potiphar's wife,
There is none greater in this house than I; neither hath he kept back any thing from me but thee, because thou art his wife: how then can I do this great wickedness [adultery], and sin against God? (Genesis 39:9)
Likewise David in doing what Joseph would not:
And David said unto Nathan, I have sinned against the Lord. And Nathan said unto David, The Lord also hath put away thy sin; thou shalt not die. (2 Samuel 12:13)
Against thee, thee only, have I sinned, and done this evil in thy sight: that thou mightest be justified when thou speakest, and be clear when thou judgest. (Psalms 51:4)
And consistent with this, the Law saith,
If a soul sin, and commit a trespass against the Lord, and lie unto his neighbour in that which was delivered him to keep, or in fellowship, or in a thing taken away by violence, or hath deceived his neighbour...Then it shall be, because he hath sinned, and is guilty, that he shall restore that which he took violently away...(Leviticus 6:2,4)
And in the NT, to sin against the brethren is to sin against Christ:
But when ye sin so against the brethren, and wound their weak conscience, ye sin against Christ. (1 Corinthians 8:12)
Meaning that what Scripture clearly teaches can be denied under the premise of esoteric elitism, with the Antinomianism that John actually countered