Early in Book Four, you are tasked with entering a stone tower. When you get to the tower there is an obnoxious politician and a few henchman trying to break in. Crow settles nearby some stairs leading down to the tower. When you stand next to Crow you can command him to fly toward the politician, which will cause him to become angry at Crow. After the first time, the politician will start throwing rocks at Crow whenever he flies nearby. If you send Crow straight at the politician a second time he'll actually hit Crow with a rock, killing him, and unlocking the achievement (the game will go to white momentarily and reload with Crow alive again).
This achievement is missable.