Yes. Though it would be hard to just out-compute the hashes without the community noticing. Just to put it into perspective: currently the network is crunching away at 16344 TeraFLOP/s ( http://bitcoinwatch.com/ ).
Yes, but destroying BitCoin would not make you any money. At least I don't see how.
Destroying BitCoin provides you lots of value if you have a vested interest in some thing that BitCoin threatens the value of.