You really are confused, you don't even know what to believe, And I have no idea what you believe either, so I can't help you. It sounds like you need some professional counsel, to help you figure out what you believe.
I’m not confused and I know what I believe. David found mercy with the Lord because he told Nathan that he had sinned against the Lord. David was forgiven of his sin. (KJV 2 Samuel 12:13) David is described in 1 Kings 11:34 as keeping the commandments and statutes of the Lord.
I know God is faithful and just to forgive us our sins and cleanse us from all unrighteousness according to 1 John 1:9.
You said that you “know that God’s people are not perfect, we still sin and commit murder and adultery and every other kind of sin.”
Christians sometimes commit sins as you mentioned. We must confess our sins to be cleansed from all unrighteousness. David confessed his sins and was forgiven by God. (KJV Romans 4:7) “Saying, Blessed are they whose iniquities are forgiven, and whose sins are covered.”
God’s people are to be holy and without blame. (KJV Ephesians 1:4) “According as he hath chosen us in him before the foundation of the world, that we should be holy and without blame before him in love:”
Titus says that we should deny ungodliness and worldly lusts. (KJV Titus 2:12) “Teaching us that, denying ungodliness and worldly lusts, we should live soberly, righteously, and godly, in this present world;”
Those unrepentant adulterers and murderers mentioned in 1 Corinthians 6:9-10 and Revelation 21:8 will not inherit the kingdom of God. They are not God’s people.
I believe it is possible for a past murderer or adulterer who has repented and been cleansed from all unrighteousness by the Lord Jesus Christ to inherit the kingdom of God.
(KJV Hebrews 12:14) “Follow peace with all men, and holiness, without which no man shall see the Lord:"