I'm in a situation that a hostage taker want to kill his lawyer and I wanna take him out with a headshot..

Take your time, line up your shot by pressing and holding the fire button. It makes the shooter hold the breath and gets you better aim at the head. You should kill the hostage taker with one shot instantly. But sometimes if your weapon ammo isn't upgraded enough, then you need to fire two shots back to back immediately to get a kill.