John Gill Commentary 1 Kings 2:31

"And the king said unto him, Do as he hath said, and fall upon him, and bury him; that thou mayest take away the blood, which Joab shed without cause, from me and from my father`s house." — 1 Kings 2:31 (ASV)

And the king said to him, do as he has said, and fall upon
him, and bury him
Let him die where he is, slay him upon the spot, and then bury him; not by the altar, but in his own sepulchre, as later related; that is, give orders to bury him there; for Benaiah being a priest, could not be concerned in the burial of him, and besides it was below the dignity of his office:

See Gill on 1 Chronicles 27:2 where Gill advances reasons for Benaiah not being a priest. Editor.

that you may take away the innocent blood, which Joab shed, from
me, and from the house of my father ;
which had been too long connived at, and had called for vengeance; and now here was a proper opportunity upon fresh sins committed to avenge it, and so remove the guilt, which lay upon him and his father's house, for not inflicting deserved punishment on him for it.