These six things the Lord hates, Yes, seven are an abomination to Him:
A proud look, A lying tongue, hands that shed innocent blood, a heart that devises wicked plans, feet that are swift in running to evil, a false witness who speaks lies, and one who sows discord among brethren. Prov 6:16-19
What is the worst sin?
Sin is sin, according to the writing of James: For whoever shall keep the whole law, and yet stumble in one point, he is guilty of all. For He who said, “Do not commit adultery,” also said, “Do not murder.” Now if you do not commit adultery, but you do murder, you have become a transgressor of the law. James 2:10-11
Transgressing the law is sin.
Paul mentioned many other things: adultery, fornication, uncleanness, lewdness, 20 idolatry, sorcery, hatred, contentions, jealousies, outbursts of wrath, selfish ambitions, dissensions, heresies, 21 envy, murders, drunkenness, revelries, and the like; Gal 5:19-21
John wrote in Revelation that the cowardly, unbelieving, abominable, murderers, sexually immoral, sorcerers, idolaters, and all liars shall have their part in the lake which burns with fire and brimstone, which is the second death.” Rev 21:8
Some sin is specified and not everyone commits it. For instance, as James said, I’m not a murderer or adulterer but I am a liar!
That’s why I think Proverbs talks about six things God hates and seven are an abomination. Those seven things have the ability to affect each of us and do affect each of us.
It keeps me humble and thankful for what the Lord Jesus did for me!
But to answer the question, the worst sin is the unrepented one!