For a TKD fighter, to block a backthrust and react with a countermove can be easily done.that simply a back thrust, not a "spiral kick. and actually it isn't really "lethal", to quote you. it's very easy to dodge or block a back thrust, simply because it takes almost three quarters of a second for the opponent to turn and then thrust out. and it's easy to miss, especially after fighting for 5 minutes.. its VERY tiring to spar, kumite (sparring) in karate (i dunno about other martial arts) are limited to one minute bouts.
However, on the streets itself, to the uninitiated or novice in martial arts, backthrust CAN be a deadly move.
I'm not talking about half-assed backthrusts, I'm talking about those where u really swing and extend ur leg fully with power. A single blow will deliver enough force to induce possible cardiac arrest. There was a case many years ago where a simple street fight as a result of a minor scuffle resulted in a fatality cos that fella used a backthrust and induced cardiac arrest in the victim.
What happened after that is hazy in my memory. But this article appeared in The New Paper many years ago.