I don't understand where there is a kill switch when you can just turn off your vehicle using the key? Besides killing the engine will mean no power to the brakes and steering as well.
Motorcycle kill switches are always on the handlebars, so you can reach it without letting go of the handlebars. Also makes it easier to find - some bikes have the keyswitch on the side, but the kill switch is always on the handlebars.
The brake booster will hold some vacuum for a few brake applications and, if the car remains in gear, the rotation will continue to create vacuum to recharge the brake booster (and turn the power steering pump, but power steering is not needed in practice at anything approaching “runaway car” speed).