Seagal Sensei said something like O Sensei's ukes were afraid to die while attacking him.
And I read a post here in aikiweb of Tim Haffner, that said:
Also, I consider when Admiral Takeshita arranged a demonstration befoe the Emperor for O'sensei in 1938. O'sensei riginally refused saying that he would not show "false" technique to the divine Emeror. When Takeshita asked further, O'sensei said that if he was to perform "true" technique he would kill the attacker. Takeshita convinced him to perform before the Emperor, but was deathly sick up until he stepped on the mat. Yukawa, his uke, tried to compensate for the illness and attacked weakly, but O'sensei responded briskly and dislocated his shoulder. Gozo Shioda Sensei had to take ukemi for the rest of the demonstration.