Reading people say "a piece of/part of his soul" makes me think:
Piece of Gwen, Piece of Harry, Piece of Peter, piece of Flash ect... but the worst pieces.
Such as that 'good and evil on our shoulders' maybe when a person dies the evil dirt on the souls is washed away before going to the afterlife and Kindred is a product of: Gwen, Harry, and all that has died but knows Peter personally. Thus having memories and a link to him as there are many Peter has a history with that has died and some that came back from death. Even the 'worse parts of Gwen and harry loves Peter but being what it is a messed up kind of caring. Pieces of souls bonded together forming bugs and the bening Kindred that was task to torture Peter's villains in the afterlife out of hate and vengeance.