O'Sensei killed one student and seriously injured another in the same seminar. All the deaths I know of are from the original honbu dojo. I think at least two or three were from shihonage. The most serious injury I've seen in my organization was caused by hakama entanglement (totally jacked up his leg, torn something or other)...although I am quite cognizant of the high injury level in aikido. Even though 99% of aikido is cooperative these injuries still occur, usually due to a lack of control on the part of nage b/c they halfway learned technique, to include ukemi. See how I brought that back to ukemi? Full circle, baby, oh yea!
I've never heard or read about O'Sensei killing a student during a seminar. Are you sure you weren't thinking about his performance before the Emporer when his first uke got a broken arm with the first throw and Shioda had to serve as uke for the next 40 minutes? As I recall Shioda ran a high fever afterwards and was bed ridden for several days.
If you have more detail on O'Sensei killing a student, please share. I'd be very interested in learning more.
Thanks and osu!