If law is simply an agreement, let the others respect the law

In ancient Greece, law was intended to make persons better. In modernity, law only coordinates individuals, does not intend to improve them. It is only a public agreement.
Economics, however, teaches us that multi-parts agreements present an incentive to violate them secretly, in the hope that other will respect the rule, while the violator enjoys a free-ride. This a seed for corruption in modern societies.