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!!!).