unintentionally kills

This happens when a person accidentally kills another person, without intending to do so.

avenge the blood of a person who was killed

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