Jamieson, Fausset and Brown on 1 Kings 2:28

28. Then tidings came to Joab--The execution of these sentences respectively on Adonijah and Abiathar prepared Joab for his fate. Death, due to his great crimes (Nu 35:33), would long ago have been inflicted, had not his power and popularity with the army been too formidable for the old king. He now fled to the altar, which, though a recognized asylum, afforded no sanctuary to the rebel and murderer (Ex 21:14). And, as he refused to leave it, he seems to have cherished some faint hope that a religious scruple would have been felt at the thought of violating the sanctity of the place by bloodshed. Benaiah, not liking to assume any responsibility, referred the matter to Solomon, who determined that the law should take its course (De 19:13).