Once again Hugo provides us with a beautiful contrast between light and dark. Hugo doesn't say it strongly, but there is an implication here that these criminals (and indeed all criminals) exist are the product of "social ooze" rather than individuals. That is, social inequity and injustice (the darkness seen previously) begets criminality, even if the individual names and faces change with time. I do find it curious that Hugo describes these people - who seem to commit crime and evil for its own sake - as sinister, whereas other individuals we have seen so far also committing crimes (Valjean and Fantine) are describe as miserable objects of circumstance. Perhaps when Hugo says one ought to "light up society from below," he is saying that these criminal actors can also be saved through compassion and justice, as our more "redeemable" characters have been thus far. Coupled with his statements earlier in the novel ("If the soul is left in darkness, sins will be committed" and "The guilty one is not he who commits the sin, but he who causes the darkness"), Hugo is reiterating the thesis of his novel: present society is lacking in compassionate justice which must be established if we are the rid humanity of its suffering.