see if you can find the BAS relay. it's in the tail and will have a couple of reds and a couple of blacks on it. see if it looks tampered with (extra wires or splices)
unplug that with the bike running. if the bike stops, try replacing the BAS relay with the high beam relay. it's very unlikely but if the BAS relay is failed closed your killswitch won't work.
after putting the high beam relay in its place, see if the system behaves normally, if it does you need a new relay.
do you have a multimeter? because a few tests would be useful to do here.