the ecu thing is about the best way to keep it from starting, your alarm however is a different story if what you are refering to is the relay that the alarm controls. I have only heard of one alarm system that does not work like all the rest when it comes to the starter disable( relay connected to the ground when armed output). Every other alarm out there, just disconnect power from the alarm and the kill switch/relay no longer functions. the relay is wired in the normally closed position, when the alarm arms it activates the relay by showing it a ground, so if you pull power the relay will deactivate and go back to the closed position.

the alarm kill is a good idea because it takes no additional effort, however the best kill switch is totally independant of the alarm system and hooked into some odd wire that while required is way beyond the common theifs comprehension.