There's a difference between kills and KO's. I personally like stabbing the KO'd until their mouth comes open, their eyes open a little and they start bleeding, showing that they are, in fact, dead.
If I'm in debug mode, I turn on the map editor, giving me the ability to fly, wolfieisgod and then I blow people up and explode their heads while shouting "I AM YOUR GOD!!!"
My other favorite way to kill without debug mode is by throwing the knife. If the victim survives, do it again. If it's a wolf then throw, if it got in him, trip, roll over him and take the knife out. Usually a kill in three easy(ish) moves.