In order to determine what the root cause of all sin is, we can look at lists of sins quoted in scripture in order to determine the common denominator. The answer might be surprising to some, but it will certainly makes sense when looking at the various sins.
Probably the most well-known sins are those mentioned in the Ten Commandments in Exodus 20, including: Killing, Stealing, Adultery, Being a False Witness, etc.
In Leviticus 18, there are also all kinds of sexual sins listed.
Paul also covers a list of sins in 1 Corinthians 6:9-10, saying,
"Do you not know that the wicked will not inherit the kingdom of God? Do not be deceived: Neither the sexually immoral nor idolaters nor adulterers nor male prostitutes nor homosexual offenders nor thieves nor the greedy nor drunkards nor slanderers nor swindlers will inherit the kingdom of God." (Also see Romans 1:24-32)
Also in Galatians 5:19-21, Paul says,
"The acts of the sinful nature are obvious: sexual immorality, impurity and debauchery; idolatry and witchcraft; hatred, discord, jealousy, fits of rage, selfish ambition, dissensions, factions and envy; drunkenness, orgies, and the like. I warn you, as I did before, that those who live like this will not inherit the kingdom of God."
Paul further explains in 2 Timothy 3:2-4 that,
"People will be lovers of themselves, lovers of money, boastful, proud, abusive, disobedient to their parents, ungrateful, unholy, without love, unforgiving, slanderous, without self-control, brutal, not lovers of the good, treacherous, rash, conceited, lovers of pleasure rather than lovers of God..."
In addition, James says, "Anyone, then, who knows the good he ought to do and doesn't do it, sins." (James 4:17)
So what do all these sins have in common? It is loving ourselves first, meaning selfishness.
James explains that the root cause of all sin is selfishness, "When tempted, no one should say, 'God is tempting me.' For God cannot be tempted by evil, nor does he tempt anyone; but each one is tempted when, by his own evil desire, he is dragged away and enticed. Then, after desire has conceived, it gives birth to sin; and sin, when it is full-grown, gives birth to death." (James 1:13-15)
The root cause of all sin is selfishness, which is loving ourselves more than we love anyone or anything else. In Matthew 22:36-40, Jesus explains the two greatest commandments, which are: Love God above all else, and Love your neighbor as yourself.
"Jesus replied: 'Love the Lord your God with all your heart and with all your soul and with all your mind.' This is the first and greatest commandment. And the second is like it: 'Love your neighbor as yourself.' All the Law and the Prophets hang on these two commandments." (Matthew 22:37-40)
Selfishness is what these two great commandments oppose. Selfishness means loving ourselves first. However, Jesus Christ explains that we are to love God first. Loving others and ourselves is supposed to be a tie for second place.
Something that is also sinful would be loving others first, with loving God being second or third. 'Loving others first' sounds like good doctrine, but it is actually evil because it places loving God below first place.
Jesus also tells us to deny ourselves, which again reveals that selfishness is the root cause of sin. Jesus says, "If anyone would come after me, he must deny himself and take up his cross and follow me." (Matthew 16:24)
And Paul explains that, "But for those who are self-seeking and who reject the truth and follow evil, there will be wrath and anger." (Romans 2:8)