What is the significance of the quote "where civil blood make civil hands unclean"?

The key word is "civil," and the phrase "civil blood" is a paradox. Citzens of a town ought to be civil; that is, they ought to show respect for one another and get along. But too often, they aren't. They engage in civil wars and shed "civil blood," which wouldn't happen if they were really civil. This paradoxical situation exists in "fair Verona," but the following phrase "where we lay our scene," implies that it could happen anywhere. Why? Not because one side is right and the other wrong. The households are "alike in dignity," and the "grudge" doesn't belong only to one or the other. It's "ancient," beyond memory. And, as the two sides share the grudge, they also share the guilt. Both sides mutiny against the peace of the town, making their "civil hands unclean."