P-MOSFET allowed some reverse current, L7805CV destroyed. Why?
You measured -1 volt with a DVM and more than likely this would rise to -12 volts with the DVM disconnected. The -1 volt is due to the 10 Mohm input impedance of the DVM discharging the capacitor. It would take severals seconds to discharge of course but, as soon as you disconnected the DVM it would charge back to -12 volts.

Then you connected a regulator to a 12 volt reverse charged capacitor and this quite possibly caused the failure. Betweeen "LOAD" and "Gnd" you need a bleed resistor and 100 kohm should nail it.