What does 1 Kings 2:5 mean?

You also know what Joab son of Zeruiah did to me—how he killed Abner and Amasa, two commanders of Israel's army, spilling their blood in peacetime as if it were war. He stained his hands with innocent blood. 1 Kings 2:5 - Modern Text Bible

(David brings up Joab's violent actions, which were unjust and brought guilt on the nation.)