זוהר בראשית — דף רלח ע”ב

The plagues of Egypt each represent a degree of punishment corresponding to the offenses committed against God. The blood corresponds to the shedding of the children's blood, the frogs to turmoil, the lice to degradation. Each plague came to break one of the forces of Egypt's spiritual power. The divine punishment was precisely calibrated against each transgression. The secret is that divine justice is always exactly proportionate to the crime committed.