Insta-kill seems to go active after using the keybind to use aimbot(Middle Mousebutton), not sure if that's normal, but that's my fix on how to make it active, all my sniper kills are one shot with snipers(Silenced snipers usually take them down to 1-2% health) and usually 2 shots with any other gun.
P.S. Lol I was bad for a round and had insta-kill active with the UMP....nice choice but yeah, not gonna do that again.