In XIII, if you have a pistol, or don't have a weapon, and you walk up to an object (chair, beer bottle, etc.) and a fist holding a bar appears, you can pick it up, and attack someone with it (with the R button), and they will instantly die.
Also, after you kill someone, if you walk over them, you get their items (ammo, medkits, weapons, armor).
If you walk up behind a civilian and two red hands appear on your screen, in you press action (A), you will take the civilian hostage, and police (in the FBI level) will back up, because they don't want to kill the hostage (IMPORTANT: If you have a hostage terrorists will still shoot you!!!).