Harry saw Sirius duck Bellatrix’s jet of red light: He was laughing at her. “Come on, you can do better than that!” he yelled, his voice echoing around the cavernous room.
The second jet of light hit him squarely on the chest.
The laughter had not quite died from his face, but his eyes widened in shock.
It seemed to take Sirius an age to fall. His body curved in a graceful arc as he sank backward through the ragged veil hanging from the arch….
And Harry saw the look of mingled fear and surprise on his godfather’s wasted, once-handsome face as he fell through the ancient doorway and disappeared behind the veil, which fluttered for a moment as though in a high wind and then fell back into place. — Harry Potter and the Order of the Phoenix
There are a lot of unanswered questions about Sirius’s death.
Bellatrix’s curse seems to have been red, not green, and did not cause instant death. If it wasn’t an Avada Kedavra curse, then what killed Sirius?
Sirius was alive when he fell through the arch. If he hadn’t fallen through the arch would he still have died? Did the veil kill Sirius?
Did Sirius fall through the arch by accident? Did Bellatrix’s curse purposely cause him to fall through the veil? Or did the arch itself pull his body toward it, like a magnet?