Neither my 240D or 300D has a "kill" switch, unless you're referring to the
"stop" lever on the linkage under the hood. The vacuum line to stop the
engine goes to a diaphragm near the rear of the fuel injector pump. Follow
the vacuum lines under the hood until you find the one that goes to the pump
and make sure it's properly connected.
The reason the engine won't stop most of the time is a lack of vacuum at the
injector pump. You need to isolate the cause which a good deal of the time
is one of the vacuum diaphragms which lock and unlock the doors, trunk and
gas filler door.
This takes a little time to isolate the culprit but is easy to do once you
understand how the system works. There is a guy who sells a manual on eBay
explaining how to troubleshoot door lock system, etc., but I found it rather
easy to do myself once I knew how to isolate the source of the problem. Let
me know how you make out. Maybe I can help.