Let's put it this way. If I had a Phaser set to stun, and it worked 100%, I'd have no reason whatsoever to set it to kill. And I would not.
But, if the govt did not let me have a kill setting that would be infringement, if those were the Arms of the day.
In that case, I would favor a contemplative lock out, to make it a conscious, yet very illegal act, to set it to kill. An ipso facto proof of intent to kill.
But, no lock for stun, open carry etc. Lots of laws already about when and how to use stun gun or pepper spray, for example, which I'm never without.