The London Merchant was an odd contrast to The Beggar's Opera in that it seemed to portray criminal behavior as simultaneously reprehensible and yet understandable.
Barnwell is most definitely shown to be held responsible for his actions and punished accordingly, and he is never acknowledged as anything but a criminal. However, we gain a degree of sympathy for him. He is coerced into criminal acts, instead of coming up with them himself, and perhaps because of this we deem him both terrible and pitiable; both deserving of judgment and mercy.
It seems as though Lillo's message is both that criminals are guilty and that criminals are victims. A simple black-and-white view of them is incomplete; only by accepting the contradicting natures of man can we understand justice.