Any technique can be potentially deadly in my opinion. Its all just a question of placing a strike correctly. I am a karate practitioner primarily and know how dangerous straight karate strikes can be. They are not the most showy or impressive of techniques but the biggest aim of karate seems to be to get your attack from A to B as quickly and accurately as possible whilst generating as much power as possible. If I hit someone in the stomach they go down, if I hit them in the solar plexus, they go down and don't get up for a while. Hypothetically speaking, if I hit someone in a more sensitive area like the throat, neck or temple with the same amount of force from a normal punch, there is a high possibility that it will do permanent or even fatal damage. This is speaking only about punches as well. Many martial arts have some absolutely crazy techniques that could easily be fatal.

Wikipedia, in an article on karate speaks about the opinion about Gichin Funakoshi (Shotokan Karate):
"Funakoshi did not consider it unusual for a devotee to use Karate in a real physical confrontation no more than perhaps once in a lifetime. He stated that Karate practitioners must "never be easily drawn into a fight." It is understood that one blow from a real expert could mean death."