Quote:IOW, the difference between manslaughter and Murder (premeditated or Heat of the Moment)...Exactly.
To be honest, I don't remember it ever being specifically stated that this "desire to kill" is what would make the difference fake-Moody was talking about - I think that's fanon, derived from the fact (Received Wisdom from Bellatrix Lestrange) that casting an effective Cruciatus requres the desire to hurt the target. It's been suggested that that requirement extends, analogously, to all three Unforgivable Curses - the desire to hurt, the desire to kill, the desire to control - and is precisely the reason why they're Unforgivable.
On the other hand, there's also the notion floating around (which something tells me may be canonical) that the reason they're Unforgivable has something to do with their being unblockable, but given that the Imperius Curse can demonstrably be resisted by the target I'm not sure that that holds any water.