While I agree with the power thing, about the curse maybe not requiring a lot of power but only hate and intent behind it, I think Rowling herself made it a bit confusing; when she first introduced Avada Kedavra, she made it sound as if only very powerful wizards could use it.

After re-reading Prisoner of Azkaban, I was reminded of a question I always had: Harry wanted to kill Sirius during their confrontation in the Shrieking Shack. He had hatred, anger, and the intent to kill for a very brief moment. I always wondered how exactly he would have done it with his wand, if he decided to go through with it. This was before the Killing Curse was officially introduced in GoF, and Harry didn't know the incantation. Could he have produced it out of pure anger, much like he blew up his aunt? Or would he have used another method that caused physical damage? You could argue that his inner goodness would never have allowed him to kill (which is true, since he couldn't bring himself to do it). So do you really think he could have potentially killed at that moment, inner goodness aside?

And although I agree that people like Bellatrix liked causing pain, I also thing that they all possessed the Slytherin quality of self-preservation. During a hardcore fight with powerful wizards, I'd think Death Eaters would rather kill and save themselves than mess around. I just think Rowling shouldn't have made it quite as easy to keep repeatedly firing Avada Kedavra. A small complaint, I know.