The yellow/red wire should be attached to a small shield-shaped phenolic contact board that slips into the groove just behind the starter button. The flat metal piece that's screwed into the body of the switch is a ground plate: if you soldered the yellow wire to it it's no wonder the bike tries to start by itself. Take that little plate off and you'll find the contact board beneath it; that's where the wire should go (and there is usually a pretty large glob of solder on the back of it. Just get the solder already there to flow, then stick your wire into it).
Assembly is button/spring in place first, then slide the contact board into its groove with the contact side facing the button, followed by the little metal ground plate. The plate has a bent tab with the hole in it that is inserted with the tab facing the button; it sits between the button and contact board and the spring butts up against it on the button side. When you push the button, the spring/plate/button stud combination grounds the starter circuit coming off the yellow/red wire.
The kill switch doesn''t keep the starter from working, it just cuts power to the coils so that the engine won't run.
Good luck
