They go to full throttle because diesels do NOT have a intake butterfly valve like a gas engine does with a throttle-body or carburetor (they are open all the time) and with a continuous supply of air and hot engine oil, ( which a diesel can burn in the same manner as fuel) they run at a very high speed and difficult to stop, ( hence ” runaway”)
the only safe way to stop them is cut off the intake air, ( by covering it up or plugging it, ) or using a CO2 extinguisher in the intake port. cutting wires, turning off ignition, or cutting fuel lines will NOT stop a runaway diesel.
they will run-on as long as there is oil in the crankcase left feeding it. The intense smoke is because of the high oil viscosity and contaminants that engine oil has versus diesel fuel.