In chapter eight, of Mary Shelley's Frankenstein, Victor is speaking with Elizabeth and Justine about Justine's guilt as she awaits to be hung for the murder of William Frankenstein. Victor, agonizing about William's death and Justine's guilty verdict, metaphorically speaks of his anguish.
But I, the true murderer, felt the never-dying worm alive in my bosom, which allowed of no hope or consolation.
Victor knows that the only reason Justine is facing death is because he was able to reanimate life; his creature was "born" and set free to wreak havoc upon the earth.
As the creature's creator, Victor is responsible for the actions of his "son" (or so he thinks). This is supported through the fact that Victor claims to be the "true murderer."
The worm symbolizes the thing which is eating away at Victor. Like the worm, his secret is buried. The worm symbolizes the thing which traverses Victor's inside and eats away at his soul. It is at this point where Victor truly comes to understand his actions. The worm is kept alive by feasting on Victor's guilt (something that will never go away). Given that Victor's guilt is eternal, the worm will always have something to feed upon (illustrating the "never-dying" worm which lives in Victor's heart and soul).