Ross Ulbricht had his laptop snatched by an undercover FBI agent while he was using it. This kill cord might have saved him some grief.
Indeed. Better, perhaps, would be to trigger wiping the LUKS header and deleting the boot partition.
The NSA laptops have two buttons on one side of the machine, to destruct crypto keys, one needs to open a cover and press two buttons simultaneously. It's a pretty good self-destruction button. But you cannot find it in your laptops.
Or perhaps you can design your kill switch like the Russian nuclear Dead Hand - the automatic nuclear retaliation mechanism is only armed if a safety switch has been explicitly switched on, in peacetime, the switch is turned off to avoid an accidental nuclear apocalypse. But remember to arm the switch every time you travel with your laptop became a question.