Those who live to gratify the sinful nature go to hell. Now remember, St. Paul is writing to Christian congregations, to groups of Christians. He knows that there are false Christians in the Galatians congregations, hypocrites who claim the name of Christian but who live oppositely, people who are baptized and go to church and yet live according to their lusts instead of actively struggling against them. And so the apostle delivers this fearful warning. When we give in to the desires of our sinful nature, when we live according to the flesh, we do so at the peril of our faith and our eternal salvation.
Sin is serious.
Sin is slavery, the loss of our freedom. Anyone who sins is a slave to sin. When we obey the urge to sin, we are no longer free. We are back in slavery. But those who belong to Christ Jesus have crucified the sinful nature with its passions and desires.