My 01suzuki marauder VT 800 stalls on the down shift as if I hit the kill switch what can cause this any suggestions
Hard to say, as it could be any number of things. My best guess would be the sidestand switch. Your left foot is shifting, which is kicking down. It's possible your foot is catching the sidestand, or the force on the peg is affecting it. If the sensor detects that the sidestand is down while the bike is not in neutral, it will kill the engine. Usually, this is by cutting off the ignition circuit, so ignition coils and fuel injection stops.
I would inspect the sensor, and if it looks ok, try temporarily bypassing the switch and see if the problem persists.