The Bible is clear that all men since Adam are under a curse. From the moment that Adam disobeyed God, he was under the judgment of sin and death. Because all men are descended from Adam, all human beings have inherited this curse.
We are not sinners because we sin, we sin because we are sinners.
The propensity towards sin comes from the fact that we are inherently flawed in our basic spiritual DNA. We commit sin because that is what we do best. It is not really possible for any human being to not sin. Although we may try very hard to be a better person, at some point we will always fail. This is true for those who do not know Jesus as their Lord and Savior, as well as those who do.
The advantage that a person who has come into a relationship with Jesus possesses, is that Jesus is now dwelling within us, giving us the power to overcome sin. Before a person comes into a relationship with the Lord, the Bible describes this person as being “slaves” to sin. After we come into this new relationship with Jesus we can become slaves to righteousness.