There's a distinction made between sin and sins in Romans. 'Sins' refers to the multiple instances of sin, but sometimes 'sin' (not just 'a sin') speaks of the sin nature rather than a specific sin. A couple of examples...
Therefore, just as through one man sin entered the world, and death through sin, and thus death spread to every person, because everyone sinned. (Romans 5:12)
Moreover the Law entered that the offense might abound. But where sin abounded, grace abounded much more, so that as sin reigned in death, even so grace might reign through righteousness unto eternal life through Jesus Christ our Lord. (Romans 5:20-21)