With regards to 3) I don't think it's the hate behind it, but to quote Bellatrix "You have to mean it" (at least I think that was Bellatrix)...

And because Snape had already arranged Dumbledore's death with him, he did mean it. He knew if Dumbledore hadn't died at his hand, he could possibly have been mutilated at another, or even, Draco could have been mutilated.

I don't think it's the power, but more the state of mind. Death Eaters are all for the kill, it's been instilled into them about this supremacy, and also they'd be scared not to use it I think, thus they have it in them, through fear and need to be able to conjour (sp?) the Avada Kedavra spell.

The Order, on the other hand, are prepared to kill because they despise everything the Death Eaters stand for. They know it's wrong and they know the harm that the Death Eaters have caused to their friends, allies, and often families-it's like a revenge.