avenge the blood of the person who was killed

Here the shed blood of a person represents their death. This can be stated in active form. See how you translated this in Joshua 20:3. Alternate translation: "avenge a person's death" (See: rc://en/ta/man/translate/figs-metonymy and rc://en/ta/man/translate/figs-activepassive)

killed his neighbor unintentionally ... accidentally killed the person

These phrases mean the same thing.