FLY KILLER doesnt kill them istantly......why?

as insecticides are used more and more, insects have become resistant to low levels of them. That may be one reason. You can also use alchohol. A few drops of rubbing alcohol will kill the fly in a much shorter time. Lysol spray has alcohol in it. It can be a cleaner and safer (perhaps even quicker) alternative than insecticides when used inside a house. It also has the added benefit of disinfecting the fly's dead body and also the surfaces around the fly.