Ding ding ding.
51% provides an economic disincentive to use hashing power for "bad". That doesn't mean someone who doesn't mind destroying millions of dollars can't try but there is no ECONOMIC INCENTIVE to do so.
There is no economic incentive to do so
so long as you're primarily invested in Bitcoins. For example, suppose someone really wealthy concluded that Bitcoins were potentially harmful to their wealth elsewhere; it might make economic sense for them to spend a few million dollars destroying Bitcoin.