Victor is here declared insane, shifting from an inherited position of respectability to the isolated social region into which his inhuman creature was born.
In the 18th century insanity was often seen as a sign of moral failure as much as medical affliction, memorably depicted as the end result of a sinful, selfish lifestyle in English satirist William Hogarth's popular morality play The Rake's Progress.
Of all the hints we receive throughout the novel that our narrator may not be entirely reliable, this brief mention is perhaps the most damning. Victor downplays his reclassificiation, emphasising that he was "called" mad rather than admitting that he actually was. However, it is hard not to feel ambiguous about such claims from a man surrounded by unsolved murders for which he blames an enormous, superhuman monster that, mysteriously, no-one else has yet reported seeing.
Arthur Belefant's Frankenstein: the Man, and the Monster (1999) radically reinterpreted the entire story as being narrated by a madman.