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.

If they threw even a fraction of their computing power into mining I suspect that they could outpace the total output of the current miners and grab the lion's share of new coins for themeselves.  They could then simply sit on these (or even "destroy" them).  Some people may argue that this would increase the value of existing coins, but I don't think so.  If there were no freely-available new coins then the barrier to entry would become much higher for potential new adopters.  The outcome would be a smaller user base, and possibly also the ultimate a collapse of the system, except pehaps amongst those already holding coins.

There's also the 51% thing to consider.  Any big player with sufficient resources could conceivably disrupt or corrupt the block chain.