I believe the term “86” stems from a 1960 Jerry Lewis movie titled “Visit to a Small Planet”. Lewis demonstrated the term by busting an azalea plant into spontaneous flame as I recall.
It’s a lot older than that. Seems to have started in the 1920s as lunch-counter slang for an item being sold out or otherwise unavailable, then evolved into a signal for “Don’t serve this guy”, and from there it was an easy step to a reference to a customer being thrown out.
I don’t remember ever seeing the phrase used to mean killing someone. Maybe it’s been used that way in really bad pulp fiction.