You are confusing the Biblical concepts of justification and santification. Justification or salvation is when a person is considered forgiven for their past, present, and future sin because of their acceptance of Jesus as their savior. Santification is the ongoing process of being conformed to Chris’s image and the continued growth of a Christian through the power of the Holy Spirit.
Scriptures on justification:
Romans 5:1 “Therefore, since we have been justified through faith, we have peace with God because of what Jesus our Lord has done for us.”
Col 2:13-14 “When you were dead in your transgressions and the uncircumcision of your flesh, He made you alive together with Him, having forgiven us all our transgressions, having canceled out the certificate of debt consisting of decrees against us, which was hostile to us; and He has taken it out of the way, having nailed it to the cross.”
Justification means positionally we are declared forgiven having our sin debt paid for us by Christ. We have peace with God and our names are written in the Lamb’s Book of Life.
Santification is where the Holy Spirit teaches us to overcome our flesh nature. If we do fall, 1 John 1:9 says, “But if we confess our sins to Him, He is faitful and just to forgive us our sins and to cleanse us from all wickedness.”
I’d rather not going into hypotheticals, but if you must....only God knows a person’s true heart. I would wager though that a person who is capable of murder is clearly not in tune with the Holy Spirit so it’s likely that they have not been born again. However, if they are truly are one of God’s chosen then the blood of Jesus covers all sins and is capable of forgiving even a murderer. King David was an adulterer and murderer but God forgave him and considered him righteous.
Is this a once for all type of event? When the person says the "sinner's prayer" are his future sins forgiven preemptively?
What you refer to as Justification is a juridical understanding of what can be easily described as our rebirth into the Family of God... our redemption. Your description of Sanctification is another juridical understanding of simply being an active member of God's Family. I thought Protestants said Catholics made things complicated...
King David was an adulterer and murderer but God forgave him and considered him righteous.
He was forgiven and restored as a result of his confession and penance through God's Grace. Now... who follows this model today?