probably a broken or frayed wire .. try to follow the wires and inspect closely with the engine running push the kill button steady and twist/turn the wires if the motor dies you found the problem
contact oxydation may cause it also … if the kill button can be taken apart open it up and clean the contacts points
Typically, two black wires from the button -- one to an amphenol connector toward the rear of the motor, and then to the power pack, and one to mounting bolt for the coil. Check those for corrosion.
Kill circuit inside cd box can fail and otherwise work (spark) just fine. If you ground the black wire at the above stated amphenol plug (leading to cd box) you should get no spark. If cd kill circuit is bad, replace - or back off low idle stop so you can turn the throttle low enough to kill motor.
