I always thought that the statement "give the devil the benefit of law" meant that we should be especially careful to follow the law to the letter, __even__ when dealing with people that we think Deserve To Be Punished (because they're bad! They're evil!), because doing otherwise is a slippery slope towards treating other people with less than the utmost care for following the rules.
Selling diamonds is not illegal but I've heard allegations of hits being put out there.
The drug laws are bad. But a la A Man for All Seasons, we should give the devil the benefit of law; what silk road does would cut a path through every law in the country.