The avadra did backfire.

It almost certainly did not.

Beneath the moonlight glints a tiny fragment of silver, a fraction of a line... (black robes, falling) ...blood spills out in liters, and someone screams a word.

AK sheds no blood. Of course, this could have been referring to another incident, but we'be been given no clues for what that incident might be.

The Killing Curse is formed of pure hate, and strikes directly at the soul, severing it from the body.

It leaves no marks, either.

The Killing Curse reflected and rebounded and struck the Dark Lord, leaving only the burnt hulk of his body and a scar on your forehead.

It never burns anything.

(And somewhere in the back of his mind was a small, small note of confusion, a sense of something wrong about that story; and it should have been a part of Harry's art to notice that tiny note, but he was distracted. For it is a sad rule that whenever you are most in need of your art as a rationalist, that is when you are most likely to forget it.)

And Harry should have noticed the discrepancy right away, but he was all weepy and shit. I suppose you or anyone else could have the same excuse. It is, after all, well written.