Qwerty, I understand the appeal of the explanation you've come up with, but it seems rather unlikely to me. It implies not only that your kill switch is a 15 kV switch, and the kill wire a piece of HT ignition lead, but also that the operator of the machine is supposed to be in constant danger of near-electrocution whenever it is running. I suggest that some other explanation is more likely to be the correct one.
What happens if you isolate the kill wire back at the ignition module?
Bear in mind that the kill wire is supposed to ground the base end of the coil primary, so when it is grounded, the switching system cannot ever isolate it from ground (which is what triggers the spark). That would mean it could experience the peak voltage across the ignition capacitor (often called the condenser in the old days). That peaks at 150 Volts or so. You might get a small spark from that voltage, but not a blue 5 mm arc, as you get from the secondary. If you are getting a nice blue spark, as long as the one you get from the spark plug lead, from your kill wire, then it sounds as if there is an unwanted connection between the primary and secondary of your coil. This could happen, for example, if the grounded end of your coil secondary had ceased to be grounded, and was flashing over to the sometimes-grounded end of the coil primary, which is the same point the kill wire connects to. If that has happened I think you need to replace the coil, preferably before someone gets hurt.
