Ultimately, Hugo's view of human nature is a contemporary one.  Hugo suggests that the change and ability to transform oneself can be a source of great power within the human being.  It is this malleability that represents how the human being's nature can ultimately be see as one directed towards good and socially redemptive elements.  Valjean is one such example.  While he might have done bad, Hugo constructs his character to be representative of how one can transform into good.  Javert is another such example.  While he might have carried himself like "granite," his letting Valjean escape realizing a greater good is evident is an example of how the human nature of change and transformation is where goodness can lie and be present. The significance of this characterization feeds into the socially transformative vision of Hugo.  He is using such a characterization of human beings to indicate that what is can be changed into what can be or what should be.  There is little static in the human being or in the world people inhabit.  Change is possible and change can be realized both within ourselves and our world.  It is to this end where his characterization of human nature is significant.