The practice of Aikido is called sincere practice. How can you be sincere with your uke if you want to hurt him? As David said in the original post, you leave yourself in the hands of tori to help them learn a technique, you trust them. Without that trust it's not Aikido, just "human origami" as someone here called it.
I had a practice partner like that once. Fortunately for me he did not inflict any injuries on me, although he did on other people; but he is only young and I hope he will grow out of it.
Not so long ago Kolesnikov sensei did a nikkiyo on me, and it was the most painful nikkiyo I've ever experienced. But he did it for a reason. He proceeded to explain (as my eyes popped out) that such an effective technique comes from softness. He later called that method "an instant satori". And when I got up I was smiling. I think this last thing is very important, it tells you a lot about the quality of your Aikido.