“and so we will destroy the heir also.” The Syriac targum says “they will destroy” instead of “we will destroy,” and some English versions adopt that reading (cp. CJB; ESV; RSV). But there is no need to amend the Hebrew text. The woman wants to make her pretend-case to protect her son as strong as possible, so she includes that the avengers of blood knew that her only remaining son was the family’s only heir, which adds the possibility of greed to distant family members wanting the man killed, because then they could take the family land. She was hoping that added information would help get David’s support for her cause.
“so that we may kill him for the life of his brother whom he killed.” The ancient world had no police force, so it fell on the family of a person who was killed to find and kill the killer. The person who found the killer and killed him was called “the avenger of blood.” [For more on the avenger of blood, see commentary on Num. 35:19].