Locate the group of wires that connect to the "run" position on your ignition switch, route that group through the kill switch "ignition" path.
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Yeah, there is no "run" terminal of the switch. There is one terminal hot in acc/run/start, and another that's hot in run/start. (There are others too, but those are the ones used that are hot in run). The point is that there are two of them, and the switches on the market have only one.

I still don't get why the simpler approach won't work or could cause any harm.