Unless of course as an employee he had access to one.

Well, require that only pilots have access to the key then.

.. which they will promptly hand over to the engineers for maintenance purposes, just as people do with their car keys.

Create a “valet key” which lets the engineer to do what he/she needs to do but doesn’t allow to take off.

And mechanics.

Or let air traffic control diasable any plane while it’s taxiing.

I’m not sure having a remote control kill switch on an airplane is the kind of backdoor that improve security (if any backdoor can, which I’d doubt).

The current "kill switch" technology for rouge airplanes is an F-18 with a few Sidewinders.

Sounds a lot safer to me and sufficient as well.

That's dangerous. Imagine this remote control gets hacked and planes start getting disabled in mid air, or even while trying to take off.

An hard to copy ignition key makes a lot more sense.