The speaker of these words is Elizabeth, a member of Viktor Frankenstein's household. She is responding to the murder of Viktor's young brother William. The Frankensteins' innocent housekeeper, Justine, has been swiftly and unjustly executed for this crime.
Elizabeth says that before these two deaths, she was aware that vice and injustice existed, i.e., they were familiar concepts to her rational mind. However, she did not experience them as real, because they were far removed from her everyday life. But ever since the horrifying sequence of events, her viewpoint has changed. Now that "misery has come home", she feels the presence of vice and injustice very keenly. She now imagines men as "monsters thirsting for each other's blood" rather than as civilized or rational creatures. By affecting her so personally, vice and injustice have gripped her imagination: she cannot stop dwelling on them and on what they imply about human nature, whereas earlier those were "remote" concepts that she knew of only via her reason.