So we are still in our sin... just not liable for penalty? What of the "regenerated" Christian who commits murder (and please don't deny that there have been some in history)?
Christians have 2 natures. As Paul points out when one is born again, they are filled with the Holy Spirit, but the “old man” or flesh nature is still present in the believer at war with the Holy Spirit. Everyone still sins because we still have the flesh nature within us, which will remain until we get to Heaven.
And what if we give in to the "flesh nature"...?
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.