Since vampires couldn't keep their extinction secret any more humas'd panic and many people 'd start chasing them without knowing their super powers how dangerous could be.Many vampires 'd kill them without hesitating at all.Also, a lot of humans because of their fear 'd start killing other humans!Scary,ha?That's why sometimes it's great better NOT to know!

I think the Volturi would find a way to cover it up. Make it look like any photos or film had been tampered with, and kill every human who'd actually witnessed it. They'd still kill Edward though, to make an example.