pretty sure you cited the wrong verse:
Romans 4:13
For the promise that he would be the heir of the world [was] not to Abraham or to his seed through the law, but through the righteousness of faith.
ironically so, seeing how this so plainly tells us that the promise of eternal life is by faith, not by law - and explicitly declares that there is a righteousness which has nothing to do with the Law at all, but is by faith.
so righteousness does not demand being under law; in fact because the purpose of the Law is to make sin more sinful, the Law is not for the righteous but the unrighteous - what it says, it says to those it condemns.
But Romans 5:13 says "of a certainty" sin was in the world before the law - not that there is no sin without law - but that sin isn't charged to anyone when there is no law. hence our perfect salvation from sun through the death of Christ, by which we who believe also died, therefore are set free from the law.
whatever is not of faith, is sin.
murder is not of faith - it is unbelief.
Cain does not need to be under Moses in order to know that sin is sinful; he only needs to believe in the goodness of God.