As Rusty Lugnuts has said, use of a knife in a kill is a symbolic act, not an imitation of the act. There is a difference between channeled aggression in Airsoft, and psychotic acts.. 'slitting' someone's throat, even with a rubber knife, is bordering on psychotic behavior.
Good luck with the knife kill tap-out thing regardless, chances are that you'll be a far better BB-magnet, before you are collecting knife-kills. Not an easy thing to do, bringing a knife to a gun-fight, and getting away with it..