The aim of the martial artist is control. I would think, if the strike killed him/her, that there was a serious lack of control. I am quite sure that the Shihan of the dojo instills this aspect of budo in the students. One of my Renshi's past away a few months ago. He was in his forties and in great shape. He had a heart attack unexpectly after a jog. He had a cardiovascular disease. He nor anyone, including the doctor he saw the week prior, knew anything about it. Needless to say, his passing was a great loss to everyone who knew him. Back to the topic...I would not worry about being killed from training. If you feel that the students lack control, pursue another dojo. I would never give up Karate. You will be happy you did not.