@Enne @Atom For what it’s worth on this old topic, much to my frustration the M112 Kill gcode, as Marlin originally intended, is present in the firmware and is supposed to do exactly as you both describe.
It is supposed to stop all motion, turn off all heaters, and additionally shut down any power supplies.
None of that functionality works because the firmware was compiled without the necessary flags / possibly because there’s a hardware limitation in the mcu.
This is something that I brought up over here:
What I’ve done in the past is make an AC relay that everything plugs into, that self kills. I designed it to manually turn on with the push of a momentary button, it will then stay on until the firmware sends a kill command and turns it off. Anything is possible when you have the necessary firmware, programming means, and cabling, and maybe someday those will be available. I have the firmware, and a means of programming, but it would be nice to have some official cable connectors to plug in.