The grace that saves reigns over all. We are not under law but under grace, we are told. Not being under law we cannot violate law. Since sin is the transgression of the law, then we remain free. We are distinctly told that sin shall not have dominion over us.
Grace does have dominion, does reign.
Just because we are not charged with our sins against our souls, not being under law, is no reason why we should be ungrateful and commit sin. The hireling-shepherd only works for wages and when safety for himself outweighs his wages he will forsake them. Not so with the shepherd who owns the sheep. Jesus owns the sheep which he tends. The hireling fleeth, because he is an hireling, and careth not for the sheep.