If Google (or some well-resourced TLA) wanted to kill BTC (I'm not saying they do, just if they did) and they decided to get serious then I imagine that they could do so fairly easily.
A bigger risk is a governmental entity deciding to step in. A small fraction of the compute horsepower they have on hand to crack cryptosystems could be used to subvert the block chain, effectively destroying all credibility. They also have the incentive to prevent others from using Bitcoins as a payment system, for example they can't tax it.
On the other hand, someone wielding 51% of the compute power could control which blocks made it into the block chain, effectively blocking transaction unless a fee was paid to the appropriate party.