@higherpower: Sakazuki is the most "shoot first, ask questions never" character ever. Why wouldn't he immediately try to kill the giant black rage dragon roaring at him, when he's done the same to literal babies at Ohara?
Also, precog/energy sensing. He'd know exactly how dangerous Acno was the second the fight started.
Acnologia destroys the magma freak. People saying acno not destroying tenrou should know that the island was protected by one of fairy tail's ultimate defense type magic. And acno's roar seems so casual.