Why do people do things away from home that they would not do at home?
Because nobody knows them there, the inhibiting influences of parents, community, and familiar surroundings are removed. This allows the old Adam—the sinful nature—to have free course, so people act in ways they would not when at home under those restraints.
Les explains this by recalling his own basic training service days, when 17-year-old recruits did things he knew they never did in their hometowns. When asked why, they answered, “Because nobody knows me.” Les calls that the perfect answer: even an unbeliever is inhibited by surroundings, circumstances, parental influence, and community influence, but in a totally strange environment away from home where no one knows them, the old Adam has free course. That same principle explains how invading soldiers can pillage a community and rape women without it bothering them—it is the old Adam. Every one of us, before salvation, was capable of those same things, because that is where the human race is coming from. Paul is showing that the old Adam abounds with sin, but where sin abounds, God’s grace is always greater. Verse 21 presents sin (the old Adam) as a king who reigned, and grace as a king who reigns even more—two alternatives.