Another way to do it is to run the cut wires through a SPDT relay. the feed side of the cut wires goes into the common terminal and the outlet (to the source) goes to the N.C. (NOT the N.O. Terminal!). Then you add a switch which closes to ground and goes to a relay-coil wire. The other coil wire gets key-on 12 volts.
This way, the original circuit defaults to working, and if the switch fails, the car runs (unless a terminal comes loose or the relay connection between the common and N.C. terminal fails). In other words, the relay is energized to make the car not run, vs needing to energize to allow the car to run.
I have to say that I am not a huge fan of said kill switches (though I have them) because if the switch fails (been there, done that) or a terminal comes loose, the car will stall and could lead to an accident.
Paul, that is the reason that killing the starter is safer IMHO (unless you stall the car in traffic, having it not start is not usually a hazard). But peeps can bypass a starter kill (starter interlock circuit) pretty easy compared to finding where the fuel or ignition kill switch is buried.
A really savvy guy would jump wires at the CCRM for a fuel cut-off.
My two cents.