If you don't already have one, buy yourself a volt/ohm test meter.
They can cost anywhere from $8 to over $100 but the little $8 one will tell you everything you need to know as long as it has a ohms scale. (Sometime back, Harbor Freight was giving away one of their inexpensive ones for free if you bought something else.
It's a "CEN_TECH digital multimeter and it even has a setting to load test 1.5v and 9v batteries).
There is obviously a short somewhere and the somewhere would be in one of the branches that turn on when the key is turned on.
A likely candidate is the wires that run back to the rear tail light (but not the turn signals as they are not "on" until you turn them on.)
If your motorcycle has a working headlight switch on the right handlebar, turn it off and then retry the ign on test. If no heavy discharge shows, the problem is in the lighting circuit.
Turn off the Kill switch and try the ign on again. If that fixes the problem, it is likely in the power line to the ignition coil or the ignition points. It also could be the coil developed an internal short.
I think the wiring diagram below is close to what is on your motorcycle.
Happy hunting.