let's be a little bit crazy here: imagine a virus that propagates through mining software and eventually reach 51% of the network silently! By that time, the 'hacker' in control, would change/manipulate the transactions...
No, he wouldn't.
People seem to have the strangest fantasies about what someone could do with 51% network power. They can avoid transactions to be included in the blockchain (thus effectively blocking some transactions), that's about it. They can NOT change/manipulate transactions, or spend bitcoins they don't have, or create more bitcoins out of thin air, or create illegal / invalid transactions, or anything else.
now, let's suppose a hacker finds a way to know the private keys from the public keys! At the moment this is considered impossible, but.... you never know.
This would implode all the cryptocurrencies down.