You are more than likely right, the nub is the ground. If you are still using stockish rubber mounted handle bars be aware that the handlebars need to be grounded to the frame (not just the forks) .
Nit pick alert! I believe a "true" kill switch comes from the days of magneto ignitions and as you stated it "grounds" the mag coil so there is no spark. Many an aviator met an untimely demise when the "kill" switch wire was broken and the mag was hot. 200 pounds of prop and crankshaft will make a rather big gash in ones skull. Most modern "kill" switches remove power from the coil or spark control circuit.
Moral (if any) know what kind of stop switch you need to have.