One justification I liked was that AK, being "fueled by hatred", can only be cast by those who are already beyond the Moral Horizon. So it's not the murder itself that's so terrible, it's that fulfilling the prerequisites for using AK means that you are a dangerous sociopath who cannot be safely let loose in the wizarding world.
Unfortunately this doesn't cover Crucio and Imperius, which IIRC are even used by some "good guys" in canon. But I'm sure you could come up with some other fan-wank to explain them.
I like that justification too, in as much as it is the best of the possible 'fan-wank'. Even so I suspect that Lily could have pulled off an AK if she had more of a chance. She had huge reserves of magical talent and a hell of a lot of hatred. Yet she still wouldn't be a dangerous sociopath. In fact, the scariest thing about sociopaths is that they don't even need to have overwhelming hatred to do brutally nasty things. The fact that most people need to be overwhelmed by emotion before they violate mores is what distinguishes them from sociopaths.
Update: This post has also been superseded - new comments belong in the latest thread.
The second thread has now also exceeded 500 comments, so after 42 chapters of MoR it's time for a new thread.
From the first thread: