Jamieson, Fausset and Brown on 1 Kings 2:5-6

5-6. thou knowest also what Joab ... did--The insolent and imperious conduct of that general had not only been deeply offensive to the feelings (2Sa 18:5-15; 19:5-7), but calculated to bring reproach on the character, to injure the prospects, and endanger the throne of David. Passing over the injuries committed directly against himself, David dwelt with strong feelings on the base assassination of Abner and Amasa.

shed the blood of war in peace, &c.--The obvious meaning is, that in peace he acted towards them as if they had been in a state of warfare; but perhaps these graphic expressions might be designed to impress Solomon's mind more strongly with a sense of the malice, treachery, and cruelty by which those murders were characterized.