Joab and his brothers were David’s nephews, sons of his sister. It’s possible that they were essential to keeping his older brothers from turning on him.
David’s brothers wer all older than him, and so would have been elderly at the time of David’s reign. Joab was the son of David’s sister. He was as unscrupulous as he was cunning. He murdered Absalom, was complicit in the death of Uriah, murdered Abner and sided with Adonijah instead of Solomon. David told Solomon to have him executed ASAP................