Agreed, mans ability to know good from evil does allow for man's sinful nature but it also allows for man to make good choices, as in to trust the gospel message and turn it God in faith for ones salvation.
If man were only able to make bad choices then what would be the logic in Christ coming to pay a sin debt. God expects man to make choices Romans 1:18-19 and that is why we have no excuse Romans 1:20 We are expected to be able to understand that God exists and we owe our very existence to Him and thus should turn to Him in faith Act of the Apostles 17:30
Man had a free will until we fell in Adam. After Adam fell, we could not think a single spiritual thought or make a righteous decision.
Genesis 6:5 says," And God say that the wickedness of man was great in the earth, and that EVERY (without exception) imagination of the thoughts of HIS heart (needed a new one, Ezekiel 36:26) was ONLY EVIL CONTINUALLY. This shows that man was unable to perceive or do the will of God, to turn or return. This ability only comes with a new heart and a new spirit given by Christ. Ezekiel 36:26-27.
What is the result of the new heart and spirit of Christ being placed in his people?
Ezekiel 36:31 Then (after receiving a new heart and spirit) shall ye remember YOUR OWN EVIL WAYS, and doings (works/will) WERE NOT GOOD (there is none good Mark 10:18) and shall loathe yourselves in your own sight (eyes opened) for your iniquities and for your abominations.
To the point that there is none good,
Jer 4:22 For my people is foolish (No sound mind as described in 2 Tim 1:7), they have not known me, they are sottish children, they are wise to do evil, but to do good THEY HAVE NO KNOWLEDGE. (Ability to do good)
Ps 53:3 Every one of them (without exception) is gone back, they are altogether (totally, in entirety) become filthy, there is none that doeth good (but doeth your own evil ways) NO NOT ONE.
AFTER being made alive, we can now blush or be ashamed of our wickedness and lies concerning the work of the hands of God. This ability to be ashamed and confounded leads to repentance from dead works which were our decisions before the life of Christ was given to us.
Jeremiah 6:15 Were they ashamed when they committed abomination, nay, they were not at all ashamed, neither could they blush, therefore they shall fall among them that fall, at the time that I visit them, they shall be cast down (thrust out Luke 13:28), saith the Lord. We had a free fallen will to sin. Jeremiah 36:32, Not for your sakes (your evil decisions, self righteousness) do I this, be it know unto you, be ye ashamed and confounded for YOUR OWN ways (vain imaginations of being restored to righteousness and fellowship), O house of Israel.
AFTER we are made alive, then we can do the will of God because we have been given the heart and spirit of Christ that we might know and do his will. Ezekiel 36:27 I will put my spirit within you, and (I WILL) cause you to walk in MY statutes, and ye SHALL keep MY judgments (not your own vain imaginations) AND DO THEM. The ability to make a GOOD DECISION in hearing the words of the covenant and doing them (Jeremiah 11:6) AFTER receiving a new heart and spirit.