Awsome topic. Well, if there is a humane way to go I suppose Avada Kedavra wouldn't be so bad, better then crucieo'ed into madness like the Longbottoms. I think that the permission to use the killing curse was gave to the aurors as means of self defense. I mean like pyscological warfare, if the Deatheaters knew they could be Avada Kedavra'ed into oblivion then maeby there would less chance of a fight. Like the death penalty in the muggle(our) world. Sure it really isn't going to stop most Deatheaters, but mabey some.
Also, on the defense note, it gave auroers the chance to defend themselves by Avada Kedavraing their enemies instead of having to try to subdue them for a traile.
Is it a cowards cures. I guess so in the way that you would have to not to die in order to cast the spell. Its a unreversable spell, once its done the other person isn't going to come back to life any time soon. So in a way, yes, kill your enemy before they can do you in. But, if used to protect your self and others that you love, then mabey not. If you knew your enemy would not stop until you are dead then as a forcable means of protection, mabey it wouldn't be a cowardly act. It's really more of a morel than ethical issue that each person would have to answer for them selves.