Possible Identity of the Criminal (SPOILER)
Do you think that it is possible that the kidnapper was Gondo's illegitamate son from, perhaps an eveing with a prostitute. They made a point that Gondo was lower class, and may have frequented blue collar haunts and met with dangerous types in his youth. That would explain his obsession and anger for being abandoned (even if Gondo did not know about the child (now young adult).
Or would that be too cliche?