The murderer discovered
It is impossible to know for certain what tone -- whether it is shock or
relief -- that Mary Shelley intends to be conveyed by this exclamation
of Victor's. In both the 1818 and 1831 texts he almost gives himself
away, then recovers before his younger brother's incomprehension and his
father's subsequent imperious self-assurance and, rather than be open with
his family members in their crisis, closes off his knowledge and himself
from their confidence.
