OK, so I follow this guy for about 4 seconds ..I am clearly behind him, I press the "stealth kill" button, I end up hitting him with my gun. He turns around and kills me with a single pistol whip..WTH? lol this happens to me 6/10 times...Any tips?

well i fdound out last night that the only way to do a stelth kill is when ur invisible u try it on the campaign if ur in armor or normal ur just bich slap them BUT if ur invisible then u can stelth kill them